RIGID PVC EXTRUSION HANDBOOK
PVC processing dates back to the industry’s beginning in the late 1930’s and early 1940’s, when oiled litharge, white lead, and the first metal-containing liquid stabilizers ( cadmium, tin) were some early additives used in the germinating vinyl industry. Over the next twenty years PVC grew to become one of the “big three” of the United States plastics industry at the billion pound per year level of consumption---yet without rigid PVC extrusion applications, which remained essentially undeveloped up to the early 1960’s. Beginning in the mid 1960’s, the uses of rigid PVC extruded products grew at an increasing rate which even outpaced the total PVC industry growth rate, to the point that rigid vinyl extrusion now consumes a greater amount of PVC resin (almost 70%) than any other process. Primary extrusion products include pipe, conduit, siding, window profile, foam-core pipe, solid and foamed profile, and sheet, with fencing and docking among emerging newer applications.
Rigid PVC extrusion has emerged as the dominant process in the United States vinyl industry, currently accounting for almost 70% of all PVC processed, primarily for pipe but also for such building products as siding, window lineals, foamed pipe and profile, fencing, and sheet. Optimum extrusion of rigid PVC-- both product quality and cost--is imperative to insure continued growth in existing markets and to permit development of new markets versus competitive materials. Understanding the proper selection-- and use -- of additives such as impact modifiers, process aids, fillers, lubricants, and stabilizers is a crucial part of developing an optimized extrusion process. Other factors which have significant effects upon the success of extrusion include compound blending procedures, proper matching of formulation to the extruder, and understanding the behavior of the extruder itself with its proper temperature and speed settings.
The discussion will center on how these factors contribute to increased output rates while maintaining product quality.
I. RAW MATERIALS' SELECTION:
Before considering the additives used in rigid PVC extrusion, the processor’s primary material concern will be the vinyl resin itself-- the major component of his formulation.
For pipe extrusion, siding, and some profile extrusion, PVC homopolymer of medium- high molecular weight ( K value: 65-70 ) is preferred. Good quality resins are available via both the suspension and bulk polymerization processes. Other rigid extrusion processes such as foamed profile or sheet typically utilize lower molecular weight PVC homopolymer ( K value: 58-62) or copolymers of vinyl chloride with vinyl acetate, ethylene, or propylene. These resins possess lower strength properties, but better melt flow and postforming properties.
Whether resin choice is influenced by end- product requirements or processing needs,the importance of resin uniformity cannot be over- emphasized.
Since most rigid PVC extrusion is from powder blends, and the extrusion process-- especially twin screw-- is sensitive to powder blend properties of flow, compact density and bulk density, the need to be aware of bulk density variations is most important. This awareness is especially necessary with the supplier’s requirement to reduce the residual VCM content of PVC resin, which in turn can mean more severe drying cycles after a steam-stripping operation in his process. Consequently, the bulk density of PVC resin, and its particle size distribution, may be somewhat different now than previously-and the resin particle surface may be a little harder, or less absorptive to liquid additives. Many resins are much drier than before. In short, the dryblend, powder flow, and bulk density differences of current PVC resins may require subtle changes in mixing procedures and extrusion conditions to maintain product quality (more on this later). Knowing this before the resin is blended and extruded can minimize a host of production problems.
Proper quality control of all incoming raw materials, especially PVC resin, is a must. Supplier Certificates of analyses on all shipments are necessary, and spot checks of moisture, bulk density, particle size distribution(% fines), dry flow, heat stability (Metrastat) and fusion (Torque Rheometer) can be very helpful.
Once the appropriate resin is selected, the compounder is faced with a staggering number of additive ingredients from which to choose. Pure PVC combines attractive cost with excellent physical and chemical properties. Any additive will change these properties and costs, since it is used for the specific purpose of enhancing either an end-use property or a processing characteristic of the PVC product. It is not only costly, but sometimes detrimental, to use more of an additive than is necessary to accomplish the intended effect. Therefore, the compounder must have a thorough understanding of his end product requirements - physical, chemical, visual - and he also must be quite familiar with the particular extrusion equipment which will be used to produce the product: single or twin screw, die characteristics, etc.
Many rigid PVC extruded products - including some pipe, siding, window profiles, and sheet - require impact strengths beyond that attainable with PVC alone, which is a rather brittle polymer. The inclusion of a small amount of another polymer with a rubber “backbone” will, with proper dispersion into the vinyl melt, provide the shock absorbing mechanism required to enhance impact strength of the PVC product by absorbing the energy of impact (measured as ft./lbs. in the crack propagating notched Izod test per ASTM D-256-72, or as a falling weight per ASTM D-2444).
Several types of specialty polymers will furnish the necessary marginal compatibility with PVC to function as impact modifiers. Chlorinated polyethylene (CPE), acrylonitrile-butadiene-styrene (ABS), methacrylate-butadiene-styrene (MBS), and certain acrylate polymers are the generic classes which are most commonly used. More recently, ethylene-vinyl acetate (EVA) has shown promise as an impact modifier. Each type offers its unique advantages - and disadvantages - which relate to weathering, clarity, effect on melt viscosity during processing, and heat stability. For example:
Weathering Resistance - favors acrylates, CPE, EVA
Clarity & Minimal Stress Whitening - favors acrylates, MBS, certain ABS types
Impact Efficiency - favors MBS, ABS
Chemical Resistance - favors no modifier - all will detract
Since impact modifiers are more costly than PVC, naturally no more should be used than necessary to provide required impact strength for the intended application. The relationship between use level and impact strength is not linear, but follows as “S” curve, characteristic for each type of modifier (See Figure 1).
Sufficient work input, or shear, must be achieved during extrusion to arrive at the optimum dispersion of modifier particles in order to obtain the maximum benefit from the quantity used. 2-5 phr, 7-10 phr, and 12-15 phr are very typical use levels to achieve desired impact strengths for pipe, profile, and sheet applications respectively.
Rigid PVC exhibits a very high melt viscosity and a tendency to stick to hot metal surfaces, resulting in uneven flow from a die opening, melt fracture and frictional heat buildup to the point of degradation. Although proper lubrication is important, and will be discussed later, process aids can minimize these problems.
The concept of a “process aid” has contributed significantly to the development of all rigid PVC processes which depend upon shear: extrusion is no exception. The most widely used process aids are specialty acrylic polymers which are used at 1.5-5.0 phr levels to:
1. Promote early, more complete fusion of the powder formulation.
2. Contribute to uniform melt viscosity.
3. Provide smooth flow from the die.
4. Minimize die swell.
5. Increase hot strength for string-up and draw-down to smaller sizes.
Other polymers, such as alpha methylstyrene, have been used as process aids in PVC extrusion.
Single screw extrusion of rigid PVC definitely requires the use of a process aid to achieve optimum quality and production rates. Typically, 1.5-3.0 phr is sufficient. While not absolutely necessary in low shear, twin screw extrusion, the inclusion of at least 1.0-1.5 phr of a process aid definitely enhances product quality. Because of its effect on promoting powder fusion, a slightly lower barrel temperature profile may be used when running with a compound containing process aid on a twin screw extruder.
Fillers are used in some rigid vinyl extrusion formulations as extenders for the prime purpose of reducing compound costs (cost per pound), but they also are used to achieve certain desirable end-use properties, such as an increased heat distortion temperature. They are usually inert solids, taken from such classes of materials as alkaline earth metal carbonates and silicates, barytes, gypsum, alums, and even wood flour. The most commonly used chemical type is calcium carbonate which comes in a variety of forms, including ground limestone of varying particle sizes, and specially purified precipitated grades - also of varying particle sizes. Some types are surface treated with stearic acid or other additives to aid in processing, and these surface coated “soft” fillers are widely used in rigid pipe extrusion, primarily because they are somewhat less abrasive than uncoated varieties. Abrasion in twin screw extrusion is seen as screw and barrel wear.
In selecting the type and amount of filler for a rigid extrusion compound, several factors should be kept in mind:
1. How much is necessary to achieve the desired improvement in tensile
strength? Can I use less of filler A to obtain the same properties achieved
with filler B?
2. While lower cost per pound of PVC compound may be one reason to incor-
porate a filler, the cost of the filler - and finished compound cost - should be
properly computed on a pound/volume basis, taking into account the differ-
ences in specific gravities of the various fillers - and their effect upon specific
gravity of the finished PVC compound. The least expensive filler - or the
higher use level of a filler - may not translate to lowest cost per foot of
3. Arriving at a choice of type and use level based on properties and cost, what
effect will this filler have on the useful life of a set of barrels and screws,
especially in twin screw extrusion? Generally, filler levels below 5.0 phr
(common for many pipe compounds) pose no real problem, but higher filler
levels - telephone duct requiring high stiffness may containup to 30 phr filler,
will shorten significantly the life of barrels and screws. In fact, many extruder
manufacturers will negate their warranty concerning screw and barrel wear
at filler levels above 5.0 phr. Thus, if high filler levels are necessary,
replacement costs for barrels and screws should be dialed into the over-all
economic analysis of highly filled rigid PVC compounds.
Pigments (and toners) are used primarily to achieve desired aesthetic effects of the finished products. However, certain types (such as various grades of titanium dioxide) serve a very useful function as U.V. reflectors to enhance the outdoor weathering life of the product. Being very inert and resistant to hydrolysis, oxidation, and extraction, titanium dioxide offers a very permanent approach to greater outdoor U.V. stability. Any rigid PVC extruded product intended for long term outdoor exposure (siding, window profiles, etc.) normally should contain at least 8 to 12 phr of titanium dioxide. At these use levels, the effect on processing is much like a filler. Extruded pipe, most of which is intended for underground use, generally requires only 1-2 phr titanium dioxide. This amount is sufficient for the short term light stability of pipe during outdoor storage prior to use - however, if longer term storage or above-ground use is expected, the titanium dioxide content should be increased to at least 4-5 phr.
Other pigments - whether organic or inorganic - do not have any measurable effect upon the extrusion process, but there are “staining” types of chemical interactions that can occur between certain pigments and the atmosphere, or other additives. For example, many pigments are based on metals (cadmium, lead, iron, chromium, selenium, bismuth, cobalt, molybdenum, manganese) which are capable of forming colored sulfides either upon exposure to sulfur-laden industrial atmospheres or by reaction with a tin mercaptide stabilizer. Pigments also possess varying degrees of U.V. stability during outdoor exposure - exhibiting bleaching, darkening, or color drift.
A full discussion of the rather complex chemistry of pigments and their reactions is beyond the scope of this discussion, but pigment selection should be made with at least an awareness of - and evaluation for - the above potential problems.
One perplexing problem that occasionally arises during long extrusion runs, especially with opaque, pigmented or filled compounds, is plateout - a term used to describe the buildup or deposit of incompatible material on the screw, in the die, on roll take-up equipment or sizing sleeves. In time this deposit affects the surface finish of the product, and also provides sites for sticking - leading to degradation. Production down-time for clean-up is then necessary. All the factors responsible for causing plate-out are not fully understood. It is believed that minor incompatibilities of additives are partly responsible but not the whole cause. Changes in type and use levels of lubricants, fillers, pigments and stabilizers, as well as changes in processing conditions (speed, temperature) can affect the severity of plateout - for better or worse. Even the weather (high humidity) has been known to aggravate plateout! Although plateout may occur in a clear formulation, most plateout incidents seem to happen with opaque, pigmented or filled compounds. Invariably, a combination of two classes of metals are usually present in plateout - prone compounds - alkaline earths (calcium, magnesium from the filler or barium from the stabilizer) and heavy metals (most commonly titanium from titanium dioxide pigment, also lead and cadmium from stabilizers or pigments).
Several empirical approaches to the control of plateout have evolved over the years, which have shown varying degrees of effectiveness:
1. Stabilizers based on organotin derivatives have not been shown to contri-
bute to plateout, mainly due to their very high compatibility or solubility in the
PVC melt. On the other hand, salts of barium, cadmium, calcium, zinc and
lead are either insoluble or less compatible with the PVC melt and can
aggravate plateout if other conditions are right.
2. Compounds based on emulsion polymerized PVC resins (E-PVC) normally
cannot be induced to show plateout. In some cases, replacing a small
amount (10-20%) of suspension (S-PVC) resin with E-PVC will reduce plate-
out. This may be due to the presence of trace amounts of emulsifier present
on the resin, for the addition of either cationic or anionic surfactants in very
small amounts (0.1 phr) has also been shown to reduce plateout.
3. A mechanical approach to plateout control which has worked successfully
involves the use of a small amount (0.5 phr) of a silicate filler to contribute a
mild abrasive or scrubbing action on metal surfaces.
4. A small container of mineral spirits mounted above the sizing sleeve of a
vacuum cooling tank so as to permit a slow, dropwise addition of mineral
spirits on the hot vinyl profile just before entering the vacuum sizer will often
keep sizing sleeves clean.
As mentioned earlier, molten rigid PVC has a tendency to stick to the hot metal surfaces of processing equipment and also possesses a very high melt viscosity which can cause uneven flow from the die, melt fracture, stabilizer consumption from frictional heat buildup, high torque on the screws, and high back pressures. If an optimum choice of all other formulation components is made without adequate consideration for proper lubricant balance, chances for successful extrusion are minimal. Unfortunately, our understanding of the science - or art - of lubrication does not measure up to its degree of importance. The critical need to unravel the nature of lubricity is especially important in rigid PVC extrusion, since it is difficult to separate completely lubricity and processing stability factors. Lubricants as additives can be separated into these broad categories: fatty acids and alcohols, fatty acid amides and esters, metal stearates, hydrocarbon waxes and low molecular weight polyethylenes. Silicones and other more exotic materials are occasionally considered for special applications.
Additive lubricants can be classified to some extent by relating their chemistry to behavior. The terms “internal” and “external” have been widely used (perhaps too much). A more accurate classification would view lubricant behavior within a broad, continuous spectrum - from the “internal” lubricity of polar molecules (stearic acid, metal stearates, fatty acid esters) to the “external” lubricity of non-polar, long straight and branched chain hydrocarbon derivatives of paraffin oils and waxes and low mole-
cular weight polyethylenes. Between these extremes can be found a number of lubricants possessing both polar groups and long hydrocarbon chains which - depending on the ratio of these groups - offer a variety of combined external-internal lubricant behavior to rigid PVC.
A lubricant’s behavior is partly characterized by its effect on powder fusion and melt viscosity. Internal lubricants tend to promote fusion and contribute to a lower melt viscosity after fusion by reducing internal friction within the PVC melt (which is actually composed of discreet particles up to about 200 degrees C). External lubricants tend to delay powder fusion, and also migrate to the surface of the PVC melt due to their incompatibility, thereby reducing frictional drag between the PVC melt and hot metal surfaces of extruder screws, barrels and dies. It is mainly this feature which helps to contribute a good surface finish to the extruded product.
Other additives - most notably stabilizers - can contribute to lubricity in rigid PVC extrusion compounds. A portion of most lead stabilizer systems consists of normal or dibasic lead stearate, while most barium-cadmium or calcium-zinc stabilizer systems are based on fatty acid soaps of these metals. These are all lubricating type stabilizers; in contrast, organotin stabilizer are essentially non-lubricating - although some types can contribute slightly to overall lubricity. Most liquid organotin stabilizers can be considered as highly efficient plasticizers which promote fusion and reduce melt viscosity.
Rigid PVC extrusion compounds based on organotin stabilizers require about 1.5-2.5 phr of total additive lubricants, properly balanced for the type of extruder (more on this later).
Lubricant levels much beyond these extremes will result in either under - or over - lubricated conditions - seen as rough surface, sticking, melt fracture, and over-heating in the former instance, or surging, lumpiness and incomplete fusion in the latter case. Much less lubricant is needed when lead, barium-cadmium or calcium-zinc stabilizers are used.
More recently, complete lubricant packages have been introduced which combine different types of external lubricant behavior with some internal characteristics. These systems are designed specifically for rigid PVC extrusion and offer a balanced, single component approach to lubrication which can result in greater economy and simplified compounding.
For most rigid extrusion applications, once the proper lubricant balance is established,
selection is generally based on the cost versus effectiveness. In clear PVC compounds, however, the lubricant’s effect upon clarity must be considered. Metal soaps, and most external wax-type lubricants are not entirely soluble or compatible in PVC, and one usually must rely on combinations of fatty acid amides and esters, which offer better clarity. 7
Most polymers undergo oxidative and photo-initiated degradation, which can be retarded effectively during processing and end-use by antioxidants.
Polyvinyl chloride (PVC) and its copolymers represent a special case in stabilization technology, since they are particularly heat sensitive. Dehydrochlorination plays an equal if not more important role than oxidation in the overall polymer degradation sequence. A multitude of unique stabilizer systems has been developed to retard PVC degradation both during the processing and subsequent life of the vinyl product.
The total energy input which a vinyl compound experiences (especially rigid compounds) includes the shear and heat energy of mixing cycles, extrusion, fabricating (embossing, thermo-forming, laminating), scrap rework, and the heat and light energy of outdoor exposure. All contribute to PVC degradation, and the stabilizer system must furnish adequate protection at every stage during the production and service life of the vinyl product.
In view of what is known about the chemistry of PVC degradation upon exposure to the above conditions, the requirements for a complete stabilizer system are many. Absorption of hydrogen chloride, displacement of active chlorine atoms prior to hydrogen chloride evolution, disruption of double bonds, free radical scavenging, peroxide decomposition, deactivation of resin impurities or degradation by-products, and ultraviolet energy absorption are some of the chemical reactions occurring during the process of stabilization. Since appearance of color is indicative of degradation in PVC, the most reliable stability tests for extrusion involve assessing color development during dynamic processing - either on the extruder itself or in laboratory tests. Alternatively, PVC samples which have undergone dynamic processing of extrusion can be tested for residual stability or thermal abuse in a Metrastat type continuous exposure oven, and visual color or U.V.- induced fluorescence that results can be measured on a scanning reflectometer. Visible discoloration is seen with 7-8 conjugated double bonds, while a UV light can detect
3-4 conjugated double bonds.
The choice of a stabilizer depends on:
1. The requirements for processing.
2. The necessary properties for end-use.
3. Cost constraints within which (1) and (2) must be obtained.
4. Other formulation components which may interact with the stabilizer.
Any classification of stabilizers is arbitrary. They will, however, be divided into the following categories for the purpose of this review:
1. Lead Salts or Soaps - Inorganic lead stabilizers were the earliest effective heat stabilizers for PVC. Litharge (lead oxide), the first lead compound to be used commercially, was soon complimented by a growing number of inorganic and organic lead compounds - basic lead carbonate, tribasic lead sulfate, lead silicates, dibasic lead phosphite, dibasic lead stearate, normal lead stearate, dibasic lead phthalate and tribasic lead maleate. These compounds furnished inexpensive and effective stabilization to PVC. Leads are still widely used in PVC electrical wire and cable insulation due to their excellent electrical properties (low volume resistivity) and low water absorption. Pipe extrusion in Europe and many other areas outside the United States also provides a very large market for lead stabilizers, and they find some use in opaque rigid vinyl extruded and injection molded products. Sulfur staining, lack of clarity and the toxicity of lead stabilizers are drawbacks which limit their use in the U.S.
2. Mixed Metal Carboxylates - Synergistic mixtures of the Periodic Table Groups II A and II B metal carboxylates represent a number of stabilizer systems for some rigid PVC application areas - many being tailor-made for specific uses.
a.- Barium-Cadmium (Zincs) - Metal ratios can be varied to account for resin zinc sensitivity, filler content, early color or long term stability requirements, clarity, and light stability needs. Barium-cadmium (zinc) soaps also offer some degree of lubrication at the expense of clarity in rigid vinyl extrusion or calendering.
The barium component provides long term heat stability, cadmium provides good early color and light stability, while zinc offers exceptional initial color (especially in highly filled systems) at the expense of long term stability. Generally used in combination with phosphites and epoxy compounds, a well balanced barium-cadmium (zinc) stabilizer system can furnish good heat and light stability. Drawbacks of these stabilizers include the toxicity of cadmium, occasional plateout tendencies, and hydrolysis or leaching of soaps from PVC. Perhaps their biggest drawback in rigid PVC is the poor melt viscosity characteristics they impart - a sort of melt stiffening.
b.- Calcium-Zincs - These FDA sanctioned stabilizers, which are used in certain rigid PVC food contact applications, are generally used with appropriate phosphite and epoxy secondary stabilizers to enhance their overall stability performance. Metal ratios can vary depending on product requirements, and other metals (magnesium, tin, aluminum, sodium) can be added, but calcium-zinc stabilization is still marginal at best, providing minimal scrap rework capability. Film and sheet extrusion (flat die and blown film), and blow molded bottles are some of the applications requiring FDA sanctioned stabilizer systems. Hydrolysis of stabilizer components in contact with water or alcohol based products can cause hazing or “blushing” of clear PVC. This problem has limited the use of calcium-zincs in clear PVC applications. Research efforts in Europe and the U.S. currently center on development of calcium-zinc systems to replace leads in Europe and organotins in the U.S. rigid PVC markets.
3. Auxiliary Stabilizers - Stabilizers in this group are normally considered secondary stabilizers, used in combination with the primary metal carboxylate stabili-
zer to provide additional “synergism,” light stability, and chelation of degradation by-products. Epoxy resins, mixed alkylaryl phosphites, certain nitrogen-containing compounds and beta diketones are included in this group of organic stabilizers.
4. Organotins - Stabilizers in this category are based upon tetravalent tin, and are true organometallic compounds by virtue of direct carbon-to-tin bonding. Having a general formula Rx SnY(4-x) , the organotins are offered in liquid and solid form and may be further distinguished by the presence or absence of sulfur in the “Y” group.
a. Tin Carboxylates - The earliest tin stabilizers, dibutyltin dilaurate and dibutyltin maleate, are typical of non sulfur containing stabilizers. These products, and subsequent developments in tin carboxylates, provided clarity to PVC and a much lower order of toxicity than lead or cadmium containing stabilizers. One stabilizer in this group, di-n-octyl tin maleate, is sanctioned by the United States FDA for rigid PVC food contact applications. Tin carboxylates are used in rigid PVC - acetate copolymer applications, but do not offer the degree of stability required to process rigid PVC homopolymers. They do, however, exhibit excellent light stability properties and are sometimes used in combination with the sulfur containing organotins to enhance outdoor weathering capabilities of rigid PVC.
b. Tin Mercaptides - The thio-organotins were introduced in the early 1950’s and offered a considerable improvement in clarity and heat stability. Although they exhibited poor light stability, some odor, and were more costly than lead or barium-cadmium stabilizers, tin mercaptides soon gained acceptance in the United States for the difficult stabilization of rigid PVC and have been the most widely used stabilizers in rigid PVC pipe and profile extrusion, injection and blow molding up to the present.
Tin mercaptides offer a unique set of properties for rigid PVC processing: classical vinyl stabilization and antioxidant functions combined with fusion promotion and melt viscosity reduction. Melt rheology studies have shown that in addition to excellent color stability, organotin-sulfur bonded compounds furnish a lower melt viscosity in rigid PVC than structurally equivalent organotin-oxygen bonded compounds (tin carboxylates) of similar viscosity, molecular weight and compatibility. They also impart lower melt viscosities than Group II A and B metal carboxylates (barium-cadmium, calcium-zinc stabilizers). A possible explanation lies in the ability of sulfur to internally satisfy the secondary bonding capabilities of tin to a greater extent than oxygen, thus preventing secondary cross-linking or “melt stiffening” of rigid PVC or copolymers which would otherwise occur through the coordination of the tin atom with groups on the polymer chain.
Up to 1970, one particular organotin mercaptide, dibutyltin bis (isooctylthioglycolate), emerged as the dominant stabilizer in the rigid PVC industry. The quality of this stabilizer was refined over the years through improved color, purity and flash point, while later modifications of butyltin technology furnished still better color and processing stability to rigid PVC - the “high efficiency” butyltin mercaptides.
During this period, di-n-octyltin bis (isooctylthioglycolate) was determined by the United States FDA to be safe for use in rigid PVC food contact applications under conditions specified in 21 CFR 121.2602. Thus for the first time, rigid PVC with glass-like clarity and sparkle could be considered for food and beverage packaging.
The tremendous growth of rigid PVC markets, especially of pipe and profiles, coupled with the increasing use of low-shear multiscrew extruders permitting lower stabilizer levels, prompted the development of lower cost “mixed metal” barium-butyltin or calcium-butyltin stabilizer systems, which provided adequate processing stability for most low cost, twin screw extrusion formulations. Many of these, however, could not be run well on single screw extruders, thus limiting their versatility.
A significant improvement in rigid PVC stabilization technology was realized in 1970 with the introduction of methyltin mercaptides. The first methyltin stabilizer exhibited substantial cost-performance advantages over classical butyltin stabilizers in a variety of rigid PVC processes, and the most recent lower cost “reverse ester” methyltin stabilizer developments continue to show significant cost-performance advantages over both the high efficiency and mixed-metal butyltin mercaptides. These advantages include better initial color stability and color retention, higher output rates, and greater regrind latitude, obtained at lower cost. These new methyltin mercaptides are finding wide use in single and twin screw extrusion of pipe, profile, foamed profile and sheet, injection molding, and blow molding. Extremely safe to use, the new, “third generation” methyltin stabilizers possess surprisingly low acute toxicity levels - (LD50:
4500-5500 mg/kg), - lower even than the earlier methyltin stabilizer that had obtained sanction for use in food contact applications from many European authorities, including the B.G.A. of West Germany, as well as the USFDA.
The basic type of stabilizer system selected is often dictated by end-use or regulatory constraints (NSF, PPI, United States FDA, German B.G.A.). Subsequent choice of a specific stabilizer should be made with the major objective of achieving optimum cost-performance - how much processing stability is available per dollar of stabilizer cost? Or conversely, what will be the lowest cost choice to furnish the required processing stability for a particular process - including all safety factors such as regrind extrusion, power failures, and end-use stability needs? A level of stability much beyond the “necessary” level can translate to significant unnecessary costs. Laboratory tests (Brabender, mill stability, Metrastat oven stability) can furnish an indication of comparative cost-performance, but the final decision really should be based on production extrusion runs and subsequent evaluation of either residual stability, or regrind extrusion, which is also conveniently done on the Metrastat oven.
Within the organotin mercaptide group of stabilizers, the most recent “reverse ester” methyltin stabilizers have demonstrated a unique ability to impart extremely good initial color stability, and to hold that color during even 100% regrind extrusion to a much greater degree and at lower costs than were previously possible. In other words, the useful processing stability of rigid PVC - to the point of product rejection - is extended. Conversely, as stabilizer level is reduced, the decrease in stability time (seen as color development) is much less with these new methyltin stabilizers than with butyltin or mixed-metal tin stabilizers. This means that a given level of stability (necessary to run an extrusion plant including a regrind safety factor) can be achieved with less stabilizer in the compound. Typical use levels for twin screw pipe extrusion are in the 0.3 - 0.4 phr range, and 0.7 - 1.2 phr for single screw pipe extrusion. The stabilizing efficiency furnished by such methyltins, especially at lower use levels, is significant,when comparing actual performance data and a schematic comparison of methyltin and lead-stabilized pipe compounds at various equivalent cost levels. In this case, a level of stability much above the “necessary” level would add unnecessary cost to the formulation and return no real benefits.