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Processing Aids for Molding
11 Processing Aids for Molding Processing aids (or “process aids”) for closed mold processes are sometimes the key additives that make a molded product both possible and profitable. In injection molding particularly, melted resin must flow quickly from screw to mold, and parts must release from the mold quickly and cleanly. And these things must happen with polyolefin compounds that may be highly filled with fillers or other additives, which are viscous when melted. Although many processing aids are used in both injection molding and extrusion, this chapter will focus on answering questions of interest to most injection molders of polyolefins (as well as rotational molders and some blow molders, to some extent):
• What is the value of common internal lubricants and other additives for increasing injection molding productivity? (Section 11.1)
• What are some kinds of mold releases and flow modifiers? (Section 11.1)
• How can processing aids improve the molding of different difficult-to-mold products? (Section 11.2) This chapter (and the next) will take a relatively narrow view of the additives that are called “process aids,” because many additives can improve the processing of polyolefins, though this may not be their primary purpose.
11.1 Melt Flow Modification and Mold Release The overarching reason for using processing aid additives usually has more to do with reducing overall processing times and costs than with enhancing properties of the compound. Even though polyolefins are relatively easy to process, molding operations still can benefit from processing aids that decrease the viscosity of the melt by lubricating the polymer internally, or by simplifying demolding by lubricating the surface of the resin.
Thus processing aids are essentially tools that reduce the time and energy to plasticate the melt, completely fill the mold, and expediently allow a part to be extracted. Such tools are also helpful for solving processing problems that can reduce an operation’s productivity. Table 11.1 summarizes the types and uses of processing aids discussed in this chapter [11-1].
11.1.1 Melt Flow-Enhancing Lubricants and Modifiers Internal processing lubricants are somewhat soluble in the polymer and allow polymer chains in the melt to slide against one another with minimal friction. This lubrication decreases melt viscosity and reduces the screw torque and processing energy required for mixing and plastication. Lubrication also assists in the complete filling of mold cavities (though sometimes lubricants may affect the physical properties in the molded part). Some lubricants are also effective at the polymer surface, where they enhance mold release (discussed more in the following section). A number of internal lubricants can be effective at loading levels below where they start to significantly lower the molded resin’s properties. These processing aids, typically used as proprietary blended formulations, are effective at ,2% concentrations when added during compounding, or even when added as pellet concentrates to dried resin right before injection molding. Some basic families of flow-enhancing aid chemistries include:
• metal stearates (stearic acid salts or soaps), which are common internal lubricants that modify viscosity and neutralize catalysts, but which can accumulate on the part’s surface, interfering with surface treatments;
• erucamide,
oleamide, and ethylene bisstearamide (EBS), amides which provide both internal flow enhancement as well as lubrication at the resin’s surface for mold release (plus, especially for EBS, filler dispersion);
Additives for Polyolefins. DOI: http://dx.doi.org/10.1016/B978-0-323-35884-2.00011-9 © 2015 Michael Tolinski, Published by Elsevier Inc.
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Table 11.1 Processing Aids for Polyolefin Molding Processes Processing Aid Purpose
Most Common Chemistries
Primary Advantages
Possible Disadvantages
Internal melt lubrication
Metal stearates; fatty amides, ester, acids, or alcohols; polymeric agents
Increased output Reduced melt viscosity and temperature Process energy savings and reduced torque Improved mold filling
Part surface contamination possible (and reduced printability) Lower physical properties (or higher, depending on the processing aid)
Rheology control (for polypropylene)
Peroxides
Viscosity reduction Improved throughput and mold filling Narrower molecular weight distribution
Interference or interaction with antioxidants
Mold release
Hydrocarbon waxes; fatty amides or acids or low molecular weight esters
Easy part release Reduced scrap Elimination of external spray release agent
Excessive part surface accumulation and contamination May not provide any melt lubrication effect
Dispersion of fillers, pigments, fibers
Various additive packages
Improved processability of highly filled compounds Similar productivity advantages of internal lubricants Improved property enhancing effects of filler
Selection and use of processing aid package complicated to determine
• esters such as glycerol monostearate (GMS) and vegetable oils, which at high molecular weights can provide internal lubrication (and at lower molecular weight, provide surface lubrication);
• other fatty acids or alcohols, which enhance resin flow at lower temperatures and injection pressures, preventing the need to increase melt temperatures to levels that might degrade the resin;
• polymeric additives, such as silicones, fluoropolymers (e.g., PTFE), or metallocene-catalyzed polyolefin plastomers or oligomers; these are relatively expensive internal lubricants, but they tend to be heat resistant and resist migrating after processing, and tend not to reduce impact strength or other properties as much as other processing aids (and they may even improve the physical properties of the resin).
Experts have noted the benefits of lower melt temperatures and pressures made possible by these additives in molding. Along with cycle-time reductions, lower pressures may allow a smaller injection-molding machine to be used for a job. Part quality may also be improved (with fewer flow lines and stronger knit lines), with less molded-in stress. (And by no means are most of these additives limited to molding processes; Chapter 12 addresses their use in extrusion, where they have analogous benefits.) [11-3, 11-4, 11-9, 11-14]. In injection molding, advanced processing aids show the extent to which additives can improve flow and reduce processing costs. Sometimes suppliers’ evaluations of their additives’ effectiveness are performed under optimal conditions, providing maximum results. But often they present various data from real production situations, or present
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Case 11.1 Effects of Processing Aids in Molding: Three Cases in One [11-2, 11-12, 11-14, 11-23]. Problem: A lack of information about how processing aids actually improve productivity. Objective: Choosing the right additive for a specific, real-world situation. Solution: Evaluate uses of processing aids in real molding situations. The supplier literature is sprinkled with stories about the effects of processing aids in various molding situations. Although some of these stories may sound anecdotal or seem to be based on idealized processing situations, they do indicate the types of processing improvements that may be possible from small additions of processing aids. A few are summarized from the literature below: Basic injection molding: Struktol has reported how its TR 016 fatty acid metal soap/amide blended lubricant allowed an injection molder to reduce cycle times by 2% when molding coat hangers from a recycled polyolefin resin. Although this does not sound like much, the cycle-time savings increased the molder’s capacity by 50,000 parts per year and translated into reduced manufacturing costs that offset the cost of the additive. Talc-filled polypropylene: Fine Organics has reported on the effect of its Plastaid-T multifunctional processing aid on 40% talc-filled polypropylene (PP) and other highly filled polyolefin compounds. In the talc-PP compound, a 0.5% additive loading was said to improve injection-molded spiral flow test lengths by 7.5% over the compound without the additive. It also increased notched Izod impact strength by 25%, reduced mold shrinkage uniformly by 19%, and maintained modulus and hardness values to within 5% of the values of the compound without the additive. The additive also reportedly decreased viscosity in this compound more than an equal loading of calcium stearate, across shear rates from 0.1 to 1 s21. Mold release of automotive thermoplastic olefins: Axel Plastics Research Laboratories has reported part-ejection force reductions resulting from its MoldWiz INT-33LCA additive, when used at 0.3% in an automotive thermoplastic olefin (TPO) containing 10% talc. The additive enabled a 9% reduction in ejection force for test parts in a single cavity cup mold on a 90-ton injection-molding press. Moreover, in the molding of actual TPO instrument panels, another of the company’s INT-33-series additives reportedly helped reduce mold pressure by 23%, reducing the cracking (and high scrap rates) that resulted when excessive force was required to extract parts from the mold.
what the range of expected improvement could be for a given material and process. An example of the kinds of reportable benefits from processing aids is from Axel Plastics Research Laboratories. One of the company’s fatty amide/modified polymer blends is said to reduce high-density polyethylene (HDPE) melt viscosity by 12 20% at 1% loading (see also Case 11.1 for other examples of process gains) [11-8]. Alternative processing aid chemistries have the toughest arguments to make for their use, since they have less history of use than standard lubricants. For example, silicone-based processing aids, such the Genioplast materials from Wacker Chemie, are said to reduce to screw torque substantially and increase the melt flow rate of both unfilled and filled polyolefin compounds. These pelletized aids are composed of an ultra-high molecular weight siloxane polymer in a fumed silica carrier that is compatible with thermoplastics. In trials in compounded resin at 1% loading, the product reportedly reduced torque by 71% for LDPE (low-density
polyethylene), 49% for PP, and 82% for PP containing 40% CaCO3. Moreover, the silicone additive improves Charpy notched impact strength of filled polyolefins, especially when loaded at 1 5% [11-5, 11-9, 11-10]. Another alternative processing aid example is the Excerex metallocene olefin oligomer from Mitsui Chemicals. This low-melting product reportedly helps speed up resin melting and output when dry blended with polyolefin pellets, reducing screw torque, barrel temperature, and cycle time. In HDPE injection molding, for instance, 3% of the oligomer reportedly allowed the reduction of cylinder temperatures from 200 to 180°C, reducing scorching. In PP bottle blow molding, 2% of the product allowed the cylinder temperature to be lowered from 190 to 170°C, reducing cooling time from 25 to 22 s and thus increasing productivity by 14%. The product is said to have a narrow molecular weight distribution (around 4000), resulting in less stickiness and less bleed out from the resin than with conventional low weight processing aids
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such as olefin waxes. A side benefit of the torque reduction is that less energy is needed per kilogram of resin processed [11-6, 11-7, 11-22]. Controlled rheology PP: Peroxide additives supply a different kind of melt flow enhancement in their use for creating controlled rheology (CR) materials, particularly important for thin-wall injection-molding applications that require high flow (and in PP fiber production as well). These viscosity-lowering (vis-breaking) additives create free radicals by cleaving or “cracking” the longest polymer chains to reduce average molecular weight in a controlled way. This also creates a narrower molecular weight distribution and improved melt flow index (MFI) (the increase in MFI is roughly linear with increasing peroxide concentration, up to 1000 ppm). Peroxides have essentially the opposite effect of antioxidants in the resin, but in small amounts, they can provide significant processing improvements. Still, CR peroxides are tricky to use, since they interact with the antioxidant and can cause color changes [3-4, 11-17, 11-21, 11-25]. Used properly, CR peroxides supplied in freeflowing concentrates ultimately can reduce processing temperatures and cycle times for PP molding. In one cited case, PP melt temperature was reduced by about 20°C, translating into 32% decreased cycle time and 50% more molded parts per hour. Their changes to melt rheology also permit a resin to fill thinner mold sections or multicavity molds. Moreover, in random copolymer PP in particular, the narrower molecular weight distribution created by CR peroxides reportedly also improves surface appearance by reducing surface roughness [11-18, 11-21]. Selecting a proper stabilization package for a CR application is difficult because stabilizers can negate the desired effects of the peroxide, thus requiring more peroxide to be added. Low interaction stabilizers have been developed by Songwon that reportedly offer good long-term stability. These resist color production and gas fading— without much affecting the CR additive’s activity in decreasing MFI for thin-wall PP injection molding (or for PP fiber spinning) [11-25].
11.1.2 Mold Release Additives Mold release additives supply external, surface lubrication to the polymer part. Being generally incompatible with the polymer, they accumulate at the surface of the cooling part and prevent it from
ADDITIVES
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sticking to the mold or to other surfaces. They also lower the release force of semicrystalline resin parts that tend to shrink and bind in the mold, and they often eliminate the need for external sprayedon mold releases. Given the low surface friction characteristics they impart, mold release additives are related to slip additives used in extrusion, discussed in the next chapter. Some overlap with the migratory flow-enhancing lubricants mentioned above. Mold release agents are typically based on one or more of the following chemistries:
• hydrocarbon microcrystalline waxes and partially oxidized polyethylene, which, though effective, may excessively pollute a part’s surface;
• fatty acids or low molecular weight esters such as GMS (also used as an antistatic additive);
• amides such as erucamide and oleamide, which provide both internal polymer lubrication as well as lubrication at the resin’s surface. Adding other internal lubricating aids to the amides may also provide adequate mold release for a given molding situation [11-3, 11-14, 11-15]. However, dedicated mold release aids are completely insoluble in the polymer, and migrate from within the polymer to the surface (and thus they do not enhance melt flow). Molders often seek a balance of soluble, flow-enhancing lubricants and insoluble mold release lubricants to enhance both melt flow and mold release. Thus commercial processing aid products for molders are formulated by suppliers to be multifunctional at low loadings (often ,1%). These blends contain soluble and insoluble lubricants appropriate for specific molding situations. They are typically supplied as a powder or as pellet blends, or in a masterbatch for feeding at the throat of the molding machine. Evaluating the effectiveness of a mold release requires somewhat less standardized test methods than the rheological methods available for evaluating the flow enhancement of a processing aid. At best, part-ejection force can be quantified and compared for a specific trial and mold. The partejection force required for demolding a part without any mold release is quantified and compared to the force values of parts with varying amounts or type of mold release to find the least-force formulation. A mold release aid might be expected to provide
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anywhere from a 10% to 40% reduction on mold release force, depending on the test situation. Such an evaluation, though difficult to set up, may be essential for operations that rely on automation for part demolding, especially where there are concerns about a part’s consistent release from the mold, and the automation’s reliability in dealing with any inconsistency [11-15, 11-16]. Optimum additive loading rates to achieve large ejection force reductions vary according to loading, pigment or filler content, and release aid composition. Loadings for commercial aids typically fall between 0.1% and 1%, with test trials required to find the loading supplying the optimum combination of release force, processing screw output, and melt temperature. Polyolefin type also determines effectiveness. For example, the “high purity” 90% monoglyceride Dimodan HS K-A GMS from Danisco is recommended at loadings of 0.1 0.25% in random copolymer PP, 0.2 0.4% for homopolymers, and 0.3 0.5% for impact copolymers. The aid’s dense composition reportedly allows it to be used at 30 50% lower loadings to provide the same release force as less-concentrated GMS additives [11-13, 11-16]. Postmolding benefits of lubricants at the part surface have been observed, although these benefits are sometimes overlooked in the literature. For example, a thin lubricant layer allows small parts to slide against one another and thus increases their packing efficiency in boxes. A low friction surface may allow parts to be more easily used by the consumer (as when opening bottle closures). Or they may allow parts to be easily assembled by the manufacturer; for instance, Croda Polymer Additives cites a case where its IncroMold S lubricant package reduced the assembly force of a PP/HDPE assembly by 30%. Low friction surfaces also reduce the noise of parts moving against other surfaces, and resist scratches and abrasion more [11-24]. External mold releases, sprayed onto the mold and not added to the compound itself, can still be valuable tools. These solutions are water- or solvent based, and when applied condense into a semidurable, heat resistant film on the hot mold that may allow dozens of part releases or more before having to be reapplied. However, if an external agent has to be used repeatedly during a production run, it can interfere with productivity, making a mold release lubricant added to the resin itself the more cost-effective approach [11-3, 11-13].
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11.2 Special Cases for Processing Aids in Molding 11.2.1 Aids for Molding Highly Filled Compounds Processing aids are useful when high filler content in a resin reduces its melt flow. Some aids enhance filler dispersion in the matrix, as well as internally lubricating the compound. Fillers that impede melt flow include titanium dioxide and other pigments; talc, mica, glass fiber, and calcium carbonate at high loadings; and nonhalogen inorganic flame retardants such as aluminum trihydrate (ATH). However, given their slippery nature, processing aids such as fatty esters or stearic acid may weaken the bond between filler and matrix, thus reducing mechanical properties of the filled compound. And conventional lubricants such as calcium stearate may only provide less than impressive increases in melt flow rate for filled compounds. But other specialized processing additives are nonblooming, multifunctional blends composed of molecules that help connect polar filler groups to nonpolar polymer groups. Along with improving the coupling of filler and polymer, they still can reduce torque and improve mold release like regular lubricants. These processing aids could be described as filler synergists—improving melt flow while also improving mechanical properties of a filled compound; a few examples are below:
• Altana’s BYK P 4101 coated silicon dioxide compatibilizer reportedly increased the mechanical properties of 40% talc-filled PP homopolymer, for example, while also reducing screw torque and pressure [11-11].
• Croda Polymer Additives’ IncroMold additives provide internal filler and glass fiber lubrication and mold release; in one case, 0.45% additive in a 30% glass fiber, carbon black-pigmented PP automotive molding reportedly increased output by 20% and eliminated the need for a silicone spray-on mold release [11-24].
• Wacker Chemie’s silicone-based torque reducing aids (also mentioned in Section 11.1.1) have also been shown to be effective for improving the impact strength, elongation, and abrasion resistance of PE and PP compounds
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Table 11.2 Comparison of Filled and Unfilled Compounds With and Without a Silicone Processing Aida Polyolefin System
Required % of Maximum Torque
Melt Flow Rate Increase (%)
Charpy Notched Impact (kJ/m2)
Without Additive
With 1.0% Additivea
Without Additive
With 1.0% Additivea
Without Additive
With 1.0% Additivea
PP
25
13
0
n/a
n/a
n/a
PP 1 40% CaCO3
96
17
0
89
3
4
LDPE
63
18
0
n/a
n/a
n/a
LDPE 1 60% ATH
60
43
0
37
17
23
a Wacker Chemie’s Genioplast Pellet S. Source: Adapted from Ref. [11-9].
that are highly filled with CaCO3, talc, or ATH (Table 11.2) [11-9].
• For ethylene propylene diene monomer rubber blended in PP (i.e., TPOs), calcium stearate and multifunctional aids such as Plastaid masterbatches from Fine Organics reportedly reduce viscosity during processing and may even enhance certain mechanical properties such as impact strength [11-12].
• Dispersion aids and compatibilizers based on reactive phosphatotitanate chemistries are also said to reduce viscosity of filled compounds. They are compatible with nonpolar polymers such as polyolefins and are available in masterbatch form (more is said about these compatibilizers in Chapter 14) [11-1].
11.2.2 Aids for Injection Stretch Blow Molding Given the interest in using clarified PP (cPP) for injection stretch blow-molded containers, every possible factor is being scrutinized that could speed up the PP-ISBM (injection stretch blow molding) process to make it more cost competitive with PET ISBM. Indeed, the longer process times for molding the thicker walls required in PP containers is one thing that is tending to delay their greater use. Clarifying nucleators, which themselves could be categorized as processing aids, have significant effects in increasing stiffness (allowing reduced wall thickness), hastening solidification, and thus speeding up cPP molding cycles.
In the injection-molding phase of ISBM in particular, the right additive can bring processing times closer to those of PET. As reported by Milliken & Company, nucleating clarifiers for cPP can reduce ISBM preform molding cycle times by about onethird, depending on the preform thickness. These nucleators increase crystallization temperature and shorten solidification time, while providing clarity [10-10, 11-20].
11.2.3 Aids for Rotational Molding As one of the less-often-encountered molding processes, rotomolding is often neglected in the literature. Yet the process is essential for producing the largest plastic products, and rotomolders use polyethylenes far more than they do any other polymer type. The times required for sintering, densification, and cooling in the process limit rotomolding to relatively low volume parts. But the long cycle times mean that even small percentage gains from processing aids can add up into several minutes cut from each part cycle, and significant cost savings. Mold release aids based on saturated primary amide chemistry, for example, bloom to the surface of rotomoldings to create an invisible lubricating layer on the part. And other additives, such as stabilizer systems containing nonphenol (nonyellowing) antioxidants, are said to speed up densification times by expediting the removal of bubbles from the densifying polymer. This not only reduces cycle times, but lowers peak internal air temperatures and saves heat energy that would be required for longer solidification cycles, according to researchers [11-2, 11-19].
• What is the value of common internal lubricants and other additives for increasing injection molding productivity? (Section 11.1)
• What are some kinds of mold releases and flow modifiers? (Section 11.1)
• How can processing aids improve the molding of different difficult-to-mold products? (Section 11.2) This chapter (and the next) will take a relatively narrow view of the additives that are called “process aids,” because many additives can improve the processing of polyolefins, though this may not be their primary purpose.
11.1 Melt Flow Modification and Mold Release The overarching reason for using processing aid additives usually has more to do with reducing overall processing times and costs than with enhancing properties of the compound. Even though polyolefins are relatively easy to process, molding operations still can benefit from processing aids that decrease the viscosity of the melt by lubricating the polymer internally, or by simplifying demolding by lubricating the surface of the resin.
Thus processing aids are essentially tools that reduce the time and energy to plasticate the melt, completely fill the mold, and expediently allow a part to be extracted. Such tools are also helpful for solving processing problems that can reduce an operation’s productivity. Table 11.1 summarizes the types and uses of processing aids discussed in this chapter [11-1].
11.1.1 Melt Flow-Enhancing Lubricants and Modifiers Internal processing lubricants are somewhat soluble in the polymer and allow polymer chains in the melt to slide against one another with minimal friction. This lubrication decreases melt viscosity and reduces the screw torque and processing energy required for mixing and plastication. Lubrication also assists in the complete filling of mold cavities (though sometimes lubricants may affect the physical properties in the molded part). Some lubricants are also effective at the polymer surface, where they enhance mold release (discussed more in the following section). A number of internal lubricants can be effective at loading levels below where they start to significantly lower the molded resin’s properties. These processing aids, typically used as proprietary blended formulations, are effective at ,2% concentrations when added during compounding, or even when added as pellet concentrates to dried resin right before injection molding. Some basic families of flow-enhancing aid chemistries include:
• metal stearates (stearic acid salts or soaps), which are common internal lubricants that modify viscosity and neutralize catalysts, but which can accumulate on the part’s surface, interfering with surface treatments;
• erucamide,
oleamide, and ethylene bisstearamide (EBS), amides which provide both internal flow enhancement as well as lubrication at the resin’s surface for mold release (plus, especially for EBS, filler dispersion);
Additives for Polyolefins. DOI: http://dx.doi.org/10.1016/B978-0-323-35884-2.00011-9 © 2015 Michael Tolinski, Published by Elsevier Inc.
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Table 11.1 Processing Aids for Polyolefin Molding Processes Processing Aid Purpose
Most Common Chemistries
Primary Advantages
Possible Disadvantages
Internal melt lubrication
Metal stearates; fatty amides, ester, acids, or alcohols; polymeric agents
Increased output Reduced melt viscosity and temperature Process energy savings and reduced torque Improved mold filling
Part surface contamination possible (and reduced printability) Lower physical properties (or higher, depending on the processing aid)
Rheology control (for polypropylene)
Peroxides
Viscosity reduction Improved throughput and mold filling Narrower molecular weight distribution
Interference or interaction with antioxidants
Mold release
Hydrocarbon waxes; fatty amides or acids or low molecular weight esters
Easy part release Reduced scrap Elimination of external spray release agent
Excessive part surface accumulation and contamination May not provide any melt lubrication effect
Dispersion of fillers, pigments, fibers
Various additive packages
Improved processability of highly filled compounds Similar productivity advantages of internal lubricants Improved property enhancing effects of filler
Selection and use of processing aid package complicated to determine
• esters such as glycerol monostearate (GMS) and vegetable oils, which at high molecular weights can provide internal lubrication (and at lower molecular weight, provide surface lubrication);
• other fatty acids or alcohols, which enhance resin flow at lower temperatures and injection pressures, preventing the need to increase melt temperatures to levels that might degrade the resin;
• polymeric additives, such as silicones, fluoropolymers (e.g., PTFE), or metallocene-catalyzed polyolefin plastomers or oligomers; these are relatively expensive internal lubricants, but they tend to be heat resistant and resist migrating after processing, and tend not to reduce impact strength or other properties as much as other processing aids (and they may even improve the physical properties of the resin).
Experts have noted the benefits of lower melt temperatures and pressures made possible by these additives in molding. Along with cycle-time reductions, lower pressures may allow a smaller injection-molding machine to be used for a job. Part quality may also be improved (with fewer flow lines and stronger knit lines), with less molded-in stress. (And by no means are most of these additives limited to molding processes; Chapter 12 addresses their use in extrusion, where they have analogous benefits.) [11-3, 11-4, 11-9, 11-14]. In injection molding, advanced processing aids show the extent to which additives can improve flow and reduce processing costs. Sometimes suppliers’ evaluations of their additives’ effectiveness are performed under optimal conditions, providing maximum results. But often they present various data from real production situations, or present
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Case 11.1 Effects of Processing Aids in Molding: Three Cases in One [11-2, 11-12, 11-14, 11-23]. Problem: A lack of information about how processing aids actually improve productivity. Objective: Choosing the right additive for a specific, real-world situation. Solution: Evaluate uses of processing aids in real molding situations. The supplier literature is sprinkled with stories about the effects of processing aids in various molding situations. Although some of these stories may sound anecdotal or seem to be based on idealized processing situations, they do indicate the types of processing improvements that may be possible from small additions of processing aids. A few are summarized from the literature below: Basic injection molding: Struktol has reported how its TR 016 fatty acid metal soap/amide blended lubricant allowed an injection molder to reduce cycle times by 2% when molding coat hangers from a recycled polyolefin resin. Although this does not sound like much, the cycle-time savings increased the molder’s capacity by 50,000 parts per year and translated into reduced manufacturing costs that offset the cost of the additive. Talc-filled polypropylene: Fine Organics has reported on the effect of its Plastaid-T multifunctional processing aid on 40% talc-filled polypropylene (PP) and other highly filled polyolefin compounds. In the talc-PP compound, a 0.5% additive loading was said to improve injection-molded spiral flow test lengths by 7.5% over the compound without the additive. It also increased notched Izod impact strength by 25%, reduced mold shrinkage uniformly by 19%, and maintained modulus and hardness values to within 5% of the values of the compound without the additive. The additive also reportedly decreased viscosity in this compound more than an equal loading of calcium stearate, across shear rates from 0.1 to 1 s21. Mold release of automotive thermoplastic olefins: Axel Plastics Research Laboratories has reported part-ejection force reductions resulting from its MoldWiz INT-33LCA additive, when used at 0.3% in an automotive thermoplastic olefin (TPO) containing 10% talc. The additive enabled a 9% reduction in ejection force for test parts in a single cavity cup mold on a 90-ton injection-molding press. Moreover, in the molding of actual TPO instrument panels, another of the company’s INT-33-series additives reportedly helped reduce mold pressure by 23%, reducing the cracking (and high scrap rates) that resulted when excessive force was required to extract parts from the mold.
what the range of expected improvement could be for a given material and process. An example of the kinds of reportable benefits from processing aids is from Axel Plastics Research Laboratories. One of the company’s fatty amide/modified polymer blends is said to reduce high-density polyethylene (HDPE) melt viscosity by 12 20% at 1% loading (see also Case 11.1 for other examples of process gains) [11-8]. Alternative processing aid chemistries have the toughest arguments to make for their use, since they have less history of use than standard lubricants. For example, silicone-based processing aids, such the Genioplast materials from Wacker Chemie, are said to reduce to screw torque substantially and increase the melt flow rate of both unfilled and filled polyolefin compounds. These pelletized aids are composed of an ultra-high molecular weight siloxane polymer in a fumed silica carrier that is compatible with thermoplastics. In trials in compounded resin at 1% loading, the product reportedly reduced torque by 71% for LDPE (low-density
polyethylene), 49% for PP, and 82% for PP containing 40% CaCO3. Moreover, the silicone additive improves Charpy notched impact strength of filled polyolefins, especially when loaded at 1 5% [11-5, 11-9, 11-10]. Another alternative processing aid example is the Excerex metallocene olefin oligomer from Mitsui Chemicals. This low-melting product reportedly helps speed up resin melting and output when dry blended with polyolefin pellets, reducing screw torque, barrel temperature, and cycle time. In HDPE injection molding, for instance, 3% of the oligomer reportedly allowed the reduction of cylinder temperatures from 200 to 180°C, reducing scorching. In PP bottle blow molding, 2% of the product allowed the cylinder temperature to be lowered from 190 to 170°C, reducing cooling time from 25 to 22 s and thus increasing productivity by 14%. The product is said to have a narrow molecular weight distribution (around 4000), resulting in less stickiness and less bleed out from the resin than with conventional low weight processing aids
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such as olefin waxes. A side benefit of the torque reduction is that less energy is needed per kilogram of resin processed [11-6, 11-7, 11-22]. Controlled rheology PP: Peroxide additives supply a different kind of melt flow enhancement in their use for creating controlled rheology (CR) materials, particularly important for thin-wall injection-molding applications that require high flow (and in PP fiber production as well). These viscosity-lowering (vis-breaking) additives create free radicals by cleaving or “cracking” the longest polymer chains to reduce average molecular weight in a controlled way. This also creates a narrower molecular weight distribution and improved melt flow index (MFI) (the increase in MFI is roughly linear with increasing peroxide concentration, up to 1000 ppm). Peroxides have essentially the opposite effect of antioxidants in the resin, but in small amounts, they can provide significant processing improvements. Still, CR peroxides are tricky to use, since they interact with the antioxidant and can cause color changes [3-4, 11-17, 11-21, 11-25]. Used properly, CR peroxides supplied in freeflowing concentrates ultimately can reduce processing temperatures and cycle times for PP molding. In one cited case, PP melt temperature was reduced by about 20°C, translating into 32% decreased cycle time and 50% more molded parts per hour. Their changes to melt rheology also permit a resin to fill thinner mold sections or multicavity molds. Moreover, in random copolymer PP in particular, the narrower molecular weight distribution created by CR peroxides reportedly also improves surface appearance by reducing surface roughness [11-18, 11-21]. Selecting a proper stabilization package for a CR application is difficult because stabilizers can negate the desired effects of the peroxide, thus requiring more peroxide to be added. Low interaction stabilizers have been developed by Songwon that reportedly offer good long-term stability. These resist color production and gas fading— without much affecting the CR additive’s activity in decreasing MFI for thin-wall PP injection molding (or for PP fiber spinning) [11-25].
11.1.2 Mold Release Additives Mold release additives supply external, surface lubrication to the polymer part. Being generally incompatible with the polymer, they accumulate at the surface of the cooling part and prevent it from
ADDITIVES
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POLYOLEFINS
sticking to the mold or to other surfaces. They also lower the release force of semicrystalline resin parts that tend to shrink and bind in the mold, and they often eliminate the need for external sprayedon mold releases. Given the low surface friction characteristics they impart, mold release additives are related to slip additives used in extrusion, discussed in the next chapter. Some overlap with the migratory flow-enhancing lubricants mentioned above. Mold release agents are typically based on one or more of the following chemistries:
• hydrocarbon microcrystalline waxes and partially oxidized polyethylene, which, though effective, may excessively pollute a part’s surface;
• fatty acids or low molecular weight esters such as GMS (also used as an antistatic additive);
• amides such as erucamide and oleamide, which provide both internal polymer lubrication as well as lubrication at the resin’s surface. Adding other internal lubricating aids to the amides may also provide adequate mold release for a given molding situation [11-3, 11-14, 11-15]. However, dedicated mold release aids are completely insoluble in the polymer, and migrate from within the polymer to the surface (and thus they do not enhance melt flow). Molders often seek a balance of soluble, flow-enhancing lubricants and insoluble mold release lubricants to enhance both melt flow and mold release. Thus commercial processing aid products for molders are formulated by suppliers to be multifunctional at low loadings (often ,1%). These blends contain soluble and insoluble lubricants appropriate for specific molding situations. They are typically supplied as a powder or as pellet blends, or in a masterbatch for feeding at the throat of the molding machine. Evaluating the effectiveness of a mold release requires somewhat less standardized test methods than the rheological methods available for evaluating the flow enhancement of a processing aid. At best, part-ejection force can be quantified and compared for a specific trial and mold. The partejection force required for demolding a part without any mold release is quantified and compared to the force values of parts with varying amounts or type of mold release to find the least-force formulation. A mold release aid might be expected to provide
11: PROCESSING AIDS
FOR
MOLDING
anywhere from a 10% to 40% reduction on mold release force, depending on the test situation. Such an evaluation, though difficult to set up, may be essential for operations that rely on automation for part demolding, especially where there are concerns about a part’s consistent release from the mold, and the automation’s reliability in dealing with any inconsistency [11-15, 11-16]. Optimum additive loading rates to achieve large ejection force reductions vary according to loading, pigment or filler content, and release aid composition. Loadings for commercial aids typically fall between 0.1% and 1%, with test trials required to find the loading supplying the optimum combination of release force, processing screw output, and melt temperature. Polyolefin type also determines effectiveness. For example, the “high purity” 90% monoglyceride Dimodan HS K-A GMS from Danisco is recommended at loadings of 0.1 0.25% in random copolymer PP, 0.2 0.4% for homopolymers, and 0.3 0.5% for impact copolymers. The aid’s dense composition reportedly allows it to be used at 30 50% lower loadings to provide the same release force as less-concentrated GMS additives [11-13, 11-16]. Postmolding benefits of lubricants at the part surface have been observed, although these benefits are sometimes overlooked in the literature. For example, a thin lubricant layer allows small parts to slide against one another and thus increases their packing efficiency in boxes. A low friction surface may allow parts to be more easily used by the consumer (as when opening bottle closures). Or they may allow parts to be easily assembled by the manufacturer; for instance, Croda Polymer Additives cites a case where its IncroMold S lubricant package reduced the assembly force of a PP/HDPE assembly by 30%. Low friction surfaces also reduce the noise of parts moving against other surfaces, and resist scratches and abrasion more [11-24]. External mold releases, sprayed onto the mold and not added to the compound itself, can still be valuable tools. These solutions are water- or solvent based, and when applied condense into a semidurable, heat resistant film on the hot mold that may allow dozens of part releases or more before having to be reapplied. However, if an external agent has to be used repeatedly during a production run, it can interfere with productivity, making a mold release lubricant added to the resin itself the more cost-effective approach [11-3, 11-13].
133
11.2 Special Cases for Processing Aids in Molding 11.2.1 Aids for Molding Highly Filled Compounds Processing aids are useful when high filler content in a resin reduces its melt flow. Some aids enhance filler dispersion in the matrix, as well as internally lubricating the compound. Fillers that impede melt flow include titanium dioxide and other pigments; talc, mica, glass fiber, and calcium carbonate at high loadings; and nonhalogen inorganic flame retardants such as aluminum trihydrate (ATH). However, given their slippery nature, processing aids such as fatty esters or stearic acid may weaken the bond between filler and matrix, thus reducing mechanical properties of the filled compound. And conventional lubricants such as calcium stearate may only provide less than impressive increases in melt flow rate for filled compounds. But other specialized processing additives are nonblooming, multifunctional blends composed of molecules that help connect polar filler groups to nonpolar polymer groups. Along with improving the coupling of filler and polymer, they still can reduce torque and improve mold release like regular lubricants. These processing aids could be described as filler synergists—improving melt flow while also improving mechanical properties of a filled compound; a few examples are below:
• Altana’s BYK P 4101 coated silicon dioxide compatibilizer reportedly increased the mechanical properties of 40% talc-filled PP homopolymer, for example, while also reducing screw torque and pressure [11-11].
• Croda Polymer Additives’ IncroMold additives provide internal filler and glass fiber lubrication and mold release; in one case, 0.45% additive in a 30% glass fiber, carbon black-pigmented PP automotive molding reportedly increased output by 20% and eliminated the need for a silicone spray-on mold release [11-24].
• Wacker Chemie’s silicone-based torque reducing aids (also mentioned in Section 11.1.1) have also been shown to be effective for improving the impact strength, elongation, and abrasion resistance of PE and PP compounds
134
ADDITIVES
FOR
POLYOLEFINS
Table 11.2 Comparison of Filled and Unfilled Compounds With and Without a Silicone Processing Aida Polyolefin System
Required % of Maximum Torque
Melt Flow Rate Increase (%)
Charpy Notched Impact (kJ/m2)
Without Additive
With 1.0% Additivea
Without Additive
With 1.0% Additivea
Without Additive
With 1.0% Additivea
PP
25
13
0
n/a
n/a
n/a
PP 1 40% CaCO3
96
17
0
89
3
4
LDPE
63
18
0
n/a
n/a
n/a
LDPE 1 60% ATH
60
43
0
37
17
23
a Wacker Chemie’s Genioplast Pellet S. Source: Adapted from Ref. [11-9].
that are highly filled with CaCO3, talc, or ATH (Table 11.2) [11-9].
• For ethylene propylene diene monomer rubber blended in PP (i.e., TPOs), calcium stearate and multifunctional aids such as Plastaid masterbatches from Fine Organics reportedly reduce viscosity during processing and may even enhance certain mechanical properties such as impact strength [11-12].
• Dispersion aids and compatibilizers based on reactive phosphatotitanate chemistries are also said to reduce viscosity of filled compounds. They are compatible with nonpolar polymers such as polyolefins and are available in masterbatch form (more is said about these compatibilizers in Chapter 14) [11-1].
11.2.2 Aids for Injection Stretch Blow Molding Given the interest in using clarified PP (cPP) for injection stretch blow-molded containers, every possible factor is being scrutinized that could speed up the PP-ISBM (injection stretch blow molding) process to make it more cost competitive with PET ISBM. Indeed, the longer process times for molding the thicker walls required in PP containers is one thing that is tending to delay their greater use. Clarifying nucleators, which themselves could be categorized as processing aids, have significant effects in increasing stiffness (allowing reduced wall thickness), hastening solidification, and thus speeding up cPP molding cycles.
In the injection-molding phase of ISBM in particular, the right additive can bring processing times closer to those of PET. As reported by Milliken & Company, nucleating clarifiers for cPP can reduce ISBM preform molding cycle times by about onethird, depending on the preform thickness. These nucleators increase crystallization temperature and shorten solidification time, while providing clarity [10-10, 11-20].
11.2.3 Aids for Rotational Molding As one of the less-often-encountered molding processes, rotomolding is often neglected in the literature. Yet the process is essential for producing the largest plastic products, and rotomolders use polyethylenes far more than they do any other polymer type. The times required for sintering, densification, and cooling in the process limit rotomolding to relatively low volume parts. But the long cycle times mean that even small percentage gains from processing aids can add up into several minutes cut from each part cycle, and significant cost savings. Mold release aids based on saturated primary amide chemistry, for example, bloom to the surface of rotomoldings to create an invisible lubricating layer on the part. And other additives, such as stabilizer systems containing nonphenol (nonyellowing) antioxidants, are said to speed up densification times by expediting the removal of bubbles from the densifying polymer. This not only reduces cycle times, but lowers peak internal air temperatures and saves heat energy that would be required for longer solidification cycles, according to researchers [11-2, 11-19].