Recently we had an inquiry by someone who believed they were having an allergic reaction to a specific plasticizer found in some soft plastic products. We replied that it was indeed possible, but this also got us thinking: the additives used in plastics processing and manufacturing are common knowledge to those in the industry, but probably very foreign to the general public. Thus, we have compiled this round-up of common plastics additives, the reason for their addition, and some considerations about them.
Read all about it after the jump.
Pigments and Colorants
Many plastics are a translucent or clear in their native state. As can be seen by the universe of colored plastic goods you see around you, most are pigmented during the manufacturing process. A huge selection of different pigments and pigment chemistries are used, sometimes in subtle and interesting ways. For example, if a plastic is yellowed from moisture during processing, adding some blue pigment will make it appear more clear. Some strong pigments are used to help protect a plastic from UV degradation, as discussed below in the stabilizers section.
Stabilizers
Some plastics break down via oxidization reactions when heated, or in reactions catalyzed by some metals. Stabilizers are materials added to polymers to prevent its degradation from sources such as heat, UV radiation or metal. Ever wonder why plastics often fade or get brittle when left in the sun? That is them being broken down by UV radiation and/or heat.
Heat Stabilizers Are usually radical scavengers or peroxide decomposers. These shut down chain reactions that could otherwise work to “unzip” the long molecules that make up polymers. These additives are often hindered phenols, aeromatic amines, phosphites, and thioesters.
UV Stabilizers Aim to protect the plastic from damaging UV rays, like sun-block for your skin. Some simply absorb the rays using strong pigments like titanium oxide or carbon black. However, if you want colors other than black or white, a hindered amine or benzophenone UV stabilizer may be added.
Fillers & Impact Modifiers
Just as rebar is used to reinforce concrete, many fillers or additives are used to toughen (or soften) plastics. Glass fibers, silica, and some special clays can greatly improve the impact toughness and shear strength of a plastic. A category of materials called “plasticizers”, including citrates, phosphate esters, phthalates, epoxidized oils, and trimellitates can be used to make a plastic more flexible and/or softer.
Some fillers such as carbon black, carbon nanotubes, or metalized glass spheres will affect a plastic’s electrical properties in addition to its physical properties.
Flame Retardants
Although some plastics such as Polyvinyl Chloride and Polytetrafluoroethylene (aka PTFE, aka Teflon) are naturally flame retardant, many others burn fairly readily.
Since most people don’t like the idea of their belongings bursting into flame (and because of various federal regulations), many consumer plastic products include flame retardants. A wide variety of flame retardants exist, and are selected based on the chemistry of the plastic, its processing, compatibility with other additives, regulatory requirements, and intended uses. Like all things, current knowledge evolves and methods change. For example, brominated flame retardants, while historically popular, are being phased out of use in many industries due to health concerns, and are banned in some EU countries.
Nucleating & Clarifying Agents
Nucleating agents add well-dispersed “imperfections” to a polymer blend. This is often desirable, as it tends to increase the temperature of crystallization; I.E. the polymer doesn’t have to be cooled as much before becoming a rigid part from its molten state, and makes the crystal formation more uniform and subtle; I.E. the part looks clear and consistent instead of like a hazy mash of snowflakes. Sodium Benzoate is often added for this purpose, but many minerals like talc will work. Clarifiers can be thought of as a special case of a nucleating agent that also increases the clarity of the resulting blend. Dibenzylidene Sorbitol is a common chemical base for clarifiers, especially often used for processing Polypropylene. Like many other additives discussed here, the additive must have a processing temperature close to that of the plastic it will be blended with, otherwise it might break down, or mix poorly.
Process Aids
When transforming a plastic into a useful shape or product, many processing steps are often required. Drying, shredding, blending, melting, extruding/molding can be expected at least, often with many more steps for more complicated processes. Some additives are useful simply to ensure the processing goes smoothly. Waxes, Silicones, Fluorinated resins and Stearates serve as lubricants or release agents, depending on the pressures and temperatures involved in the process. Ethylene bis-stearamide and other gents are used as “slip” or “anti-blocking” additives – these prevent the surfaces of a polymer from sticking to itself, particularly when being wound onto a spool or stacked in sheets.
Antimicrobials
Antimicrobials are used to produce things ranging from medical surfaces that are more sterile to synthetic shirts that don’t stink after you sweat in them for 3 days. Zeolite Silver has inherent antimicrobial properties, and can be added in several forms. Some additives have structural antimicrobial properties such as BIOSAFE® materials which are actually “pointy” on the nano-scale in a way that ruptures bacterial cell walls. For some applications a temporary antimicrobial property may be sufficient, and the plastic may be treated post-production with a biocide like Triclosan.
Foam Blowing Agents
Foamed plastics are used for insulation, flotation, and space-filling properties (among others). Car seat padding, some mattresses, shoe soles and pipe insulation are just a few examples of foamed plastic products. Although many different processes exist to produce stable foams, many involve the addition of a Foam Blowing Agent which will vaporize or expand to form the nacelles desired; some even have additional inherent insulative properties that support the final function of the product. Many blowing agents were originally fluorocarbon based, but again as techniques and laws advanced many substitute foam blowing agents that are non-ozone-depleting now exist and are used depending on the foam properties and materials in question.
Problems with Additives
Clearly, with the many possible additives for plastics, processing can get a little complicated. Additives must be compatible with the base plastic, play nicely with each other, and be appropriate for the temperatures and conditions used in processing. In addition, most additives require careful dispersion for good results, which may even call for dispersive aids – additives for your additives!
Unfortunately, some additive problems aren’t immediately obvious, and show up later via mechanisms like blooming (additives migrating to the polymer’s surface), fading, mechanical weakness or process variability.
Like most complicated problems, unfortunately, there is no one-size-fits-all answer, and the best plan is to seek expert help, or trek down the road to becoming an expert yourself.
If you’re producing a few tons a day of some compounds, chances are you already have Dow® on speed dial – for everyone else: Bay Materials is happy to help.
