Titanium Dioxide for Plastics - Basics
Cristal's range of titanium dioxide pigments for plastics can help to optimize the performance of your end application. Select a topic below for more information on how Cristal products provide superior performance in these basic areas:
Titanium dioxide is one of the most abrasive products likely to be encountered by plastics processors. The high level of abrasion can accelerate machinery wear resulting in metal contamination and color deterioration. The use of wear-resistant components and the modification of processing techniques, which minimize the contact time between the metal surfaces and the titanium dioxide powder, will assist in reducing the effect. Titanium dioxide is often added directly into the melt stream in extrusion processes and added towards the end of the mixing cycle in dry blending operations. There are no significant differences in abrasion resistance between the range of surface treated rutile products commonly employed in the plastics industry. In some processes, the choice of titanium dioxide product may influence the incorporation time into the polymer melt and thus impact the level of abrasion.
Die lip build-up can arise in any extrusion process where a polymer melt exits the die. It occurs through the presence of low molecular weight species of the polymer and additives and via oxidative degradation of both resulting in deposits on the lips of the die. If these deposits are allowed to accumulate, they can lead to processing problems and create die lines and defects on the extrudate. This in turn leads to increased reject rate and ultimately downtime of the equipment. Cristal has products specifically designed to help reduce this issue. Please contact us for advice from one of our technical service specialists.
Titanium dioxide, like all pigments, must be dispersed to achieve maximum efficiency in use. Dispersion is the process of breaking up agglomerates of pigment particles into individual particles, wetting the individual particles out and preventing the reagglomeration of the particles. In polymer systems, the first two activities are of higher importance since the viscosity of the polymer system greatly reduces the rate of reagglomeration. In liquid systems, reagglomeration is a significant issue. Each titanium dioxide product has been designed to maximize its performance in certain applications. One of the differences between titanium dioxide pigments are the surface treatments and relative affinity those treatments have for various matrices. Cristal's range of TiO₂ products have been specifically formulated to give optimum dispersion in a wide range of polymer systems. For more information on which product to use, please contact us.
Durability and Weathering
Since pigmentary TiO₂ absorbs much of the UV light it comes into contact with, one of the more useful properties it can impart in plastics is durability. UV light, particularly in conjunction with other influencing factors of weather, can create havoc on plastics, such as in the wavelength range of 300 – 350 nanometers. Many outdoor plastics contain high levels of TiO₂ to aid in its protection. Other factors of weathering include other forms of solar radiation, oxygen, ozone, water, and industrial pollution. Cristal has many years of experience with the effects of weathering on plastics substrates. We are pleased to share our knowledge on selecting a pigment to improve the durability of your end product. Please contact us to ask our technical experts a question.
Lacing is the term applied to a processing phenomenon in high temperature extrusion processes where imperfections are observed in the film manifested as holes or lensing as it emerges from the die. Lacing occurs as a function of processing temperature and residence time, and at a high enough melt temperature, typical of high-end cast film or extrusion coating. Moisture or other volatile components in the resin or additives can phase change to gas under pressure in the extruder or die, or upon exiting the die, to form voids in the film. This may be caused by not properly drying a hygroscopic polymer or by subsequent moisture pick up prior to processing. The contribution of titanium dioxide to lacing can be minimized by using specific TiO₂ products from Cristal. For more information, please contact one of our technical service engineers.
Melt Flow Properties of Polymers with Titanium Dioxide
The melt flow properties, or "rheological" properties, of polymer compounds are critical to the design of plastics manufacturing. Polymer melts are viscoelastic, meaning they have features of both viscous liquids and elastic solids. Under an applied force, perfect viscous liquid will instantaneously flow but cannot recover. Perfectly elastic solids will deform with an applied force but can recover the deformation when the force is relaxed. Polymer melts, like other viscoelastic materials, will flow under applied force; but it will recover part of its extension when the force is relaxed.
Being an inorganic additive, titanium dioxide tends to considerably increase the viscosity of the polymer melt. Although this is desired in some applications, it is often difficult to melt process if the viscosity gets too high. Some TiO₂ products are engineered to minimize the effect on viscosity of the polymer melt and are particularly useful when one must run at a lower temperature to minimize degradation. Cristal offers titanium dioxide pigments that utilize hydrophobic surface treatments to promote wetting and compatibility with the polymer matrix. These decrease inter-particle forces so polymer molecules can slip easily between treated TiO₂ particles resulting in lower viscosity. Viscosity of the final concentrate is highly dependent on the type of hydrophobic surface treatment used.
For masterbatches/color concentrates, there are economical advantages for increasing the loading of titanium dioxide. Higher loadings decrease the amount of carrier resin to be incorporated into the final product - minimizing the effect on the mechanical/physical properties.
Yellowing essentially occurs in plastics due to the over-oxidation of a phenolic compound (typically the antioxidant) as a result of the processing conditions, environmental or chemical interactions. With the latter, the mechanisms can be complex and involve common types of antioxidants (phenolics), HALS and TiO₂ pigment. These mechanisms can be initiated during processing (thermally induced) or on exposure to UV light (photochemically induced), heat and NOX gases. The correct selection of titanium dioxide pigment can help reduce the level of yellowing. Because titanium dioxide pigment particles readily absorb UV light, less UV is absorbed by the plastics leading to the formation of fewer free radicals and colored species. Titanium dioxide pigments are photo-reactive, and they may catalyze some of these reactions. The degree in which they can participate depends on level and type of surface treatment. Silica encapsulation (uniform and continuous layer of silica) of the particle prevents interactions between the titanium dioxide surface and the additives, offering the best protection and retention of the plastics original color. Cristal is pleased to help you select the correct titanium dioxide product for your application.
Cristal has many years of experience in the plastics industry and is able to offer advice on the use of its range of pigments in many current systems and end uses. A brief description of the major end use markets in plastics is provided below. Cristal is pleased to advise you on the use of its pigments in any of the following applications. Please contact us for additional information.
Engineering plastics are those plastic resins that, with or without fillers or reinforcements, have mechanical, chemical and thermal properties suitable for use in construction, machine components and chemical processing equipment. These include ABS, acetal, acrylic, fluorocarbon, nylon, phenoxy, polybutylene, polyaryl ether, polycarbonate, polyether, polyether sulfone, polyphenylene oxide, polysulfone, polyimide, polyphenylene sulfide, thermoplastic urethane elastomers, and many other reinforced plastics.
TiO₂ is included in the engineering resin formulation to achieve desired properties such as high whiteness, hiding power, chemical stability, and weatherability.
It is important to select the correct TiO₂ product to maximize dispersion, optics, and weatherability. Your Cristal sales representative will be pleased to advise you on the selection of the correct product for your engineering plastics application.
Flexible PVC is polyvinyl chloride resin that has absorbed plasticizer (mostly phthalates) to achieve desired properties such as hardness, tensile, elongation and high brittle point. Flexible PVC is compounded with many other ingredients necessary for physical properties and processing. First and foremost, a heat stabilizer must be chosen. Other important additives are colored pigments, lubricants, fillers, protective agents, pesticides and biocides and impact modifiers.
TiO₂ is included in the formulation to achieve desired properties such as high whiteness, hiding power, color stability, and weatherability. It is important to select the correct TiO₂ product to maximize dispersion, optics, and weatherability. Calendered PVC
The calendering process is used to produce highly accurate films and sheets from PVC at a large volume, you should use the calendering process. Web thickness ranges of 0.08 – 0.8 mm can be achieved by applying pressure between two or more counter-rotating rolls. The designs of modern calender trains are similar for both rigid and flexible PVC, and the choice of TiO₂ is dependent on the desired properties. For applications where maximum dispersion is desired, a general purpose or high performance TiO₂ is recommended. For maximum weatherability, a durable TiO₂ product will ensure low chalking and maximum color hold. Plastisols
Plastisols are liquid dispersions of special PVC resins in liquid plasticizers. Stabilizers, pigments, filler, diluents, and foaming agents are also common compounding ingredients. Since plastisols are processed in liquid form, viscosity is one of the most important properties.
The choice of TiO₂ is dependent on its dispersion in the plastisol and also its affect on viscosity. General purpose or high performance TiO₂ are highly recommended in plastisols because of their consistent particle size, particle size distribution, and surface treatment application that ensures consistent dispersion and oil absorption. For maximum weatherability, durable TiO₂ will ensure low chalking and maximum color hold.
Liquid colorants have some advantages over conventional methods to color thermoplastics. Liquid colorants are extremely clean and can facilitate fast color changes between production runs resulting in excellent productivity and efficiency for the manufacturer. In this application, pigment and additives are added to an alkyd or other type of oil or pre-polymer liquid carrier with TiO₂ concentrated as high as 80%. Liquid vehicle is then pumped to an extruder, blow molder, or injection molder for conversion.
TiO₂ selection is important for liquid colorants to maximize dispersion, optics, weatherability, colloidal stability and oil absorption, and both general purpose and high performance TiO₂ are recommended. For maximum weatherability, a durable TiO₂ will ensure low chalking and maximum color hold. Please contact your Cristal sales representative for further advice on product selection.
Masterbatch/Color Concentrate - Extrusion Coating
Extrusion coating refers to the process where a flat die, similar to a cast film die, extrudes a curtain of melt which is laid down on a substrate (i.e. paperboard) where it adheres, cools, and forms a thin polymeric coating. Extrusion coating is probably the most demanding process for TiO₂ due to the very high processing temperatures (>600oF/315oC) required to permit an intimate bond between the polymer and the substrate before cooling. This process is extremely demanding of the TiO₂ and an incorrect choice can result in serious quality problems and equipment downtime. While general purpose TiO₂ may be used successfully in extrusion coatings, a high performance TiO₂ will perform more consistently in this hostile processing environment. Cristal is pleased to advise on which of the products is most suited to this application.
Masterbatch/Color Concentrate - Films
The vast majority of blown film comprises polyethylene (LDPE, LLDPE, HDPE) produced for in-line or out-of-line bagmaking. The nature of the blown film process includes minimization of melt temperature to reduce the cooling load, which is often rate limiting, and maximize melt strength for bubble stability. TiO₂ performs well for most general purpose film applications by providing excellent dispersion and less blue reflectance from thicker film and more blue transmission from thinner film. When dispersion is extremely critical or when using a very highly loaded concentrate (80% +), a high performance TiO₂ may be desirable. Be sure to review the FDA status of the TiO₂ used for food packaging applications.
When processing film for semi-durable applications (such as agricultural, mulch, construction and automotive overwrap films), a durable TiO₂ product is recommended. In addition to having the proper inorganic treatment for durability, the TiO₂ product requires an organic treatment for dispersion to provide processability and adequate UV protection.
TiO₂ requirements in cast film are somewhat similar to blown film with the exception that the process runs at much higher temperatures and throughputs due to the more efficient chill roll cooling (vs. air with blown film) and typically narrower die gaps (higher shear).
This process is less dependent upon the melt strength of the polymer, and due to the high temperatures, more prone to lacing and die lip build-up problems. These result in downtime and quality problems. For high temperature cast film (>550oF/288oC), we would advise high performance TiO₂ due to its excellent thermal stability and low die lip build-up. Printing, heat sealing, or interlayer bonding in coextruded structures should not be adversely impacted.
Masterbatch/Color Concentrate - Molding
The molding process, particularly injection molding, is the largest volume fabrication process for thermoplastics. The primary requirement is to provide pigmentation as opposed to opacification and UV stability.
In most applications, general purpose TiO₂ is recommended because of its excellent dispersion (at low concentrations), brightness and color stability. Polymers, which are inherently yellow (ABS, PEI, and acetal among others), would benefit from a blue tone TiO₂ product. Consult your Cristal technical expert for help in product selection.
A general purpose or high performance TiO₂ is the pigment of choice for this typically non-critical application. Be sure to review the FDA status of the TiO₂ product prior to use in a food contact application by contacting your Cristal sales representative.
Rigid PVC is polyvinyl chloride resin without the addition of a plasticizer added. Therefore, rigid PVC is not flexible, which is desired for it has many end uses like building products and is the material of choice for window, siding, doors, decking, fencing, and soffit products.
Rigid PVC is compounded with many other ingredients necessary for processing and physical properties. First and foremost, a heat stabilizer must be chosen. Other important additives are thermal stabilizers, colored pigments, lubricants, fillers, protective agents, pesticides and biocides, and impact modifiers.
In most outdoor applications, TiO₂ is utilized at up to 10% loadings to increase weatherability as it strongly absorbs in the UV electromagnetic spectrum effectively quenching much of this radiation and thereby protecting the PVC polymer from degradation. Two types of TiO₂ are offered. The choice depends on your preference for chalking and or colorhold. Chalking TiO₂ products are not encapsulated with amorphous silica or increased levels of alumina, silica or zirconium. After absorption of the UV radiation, electrons and holes free radicals are generated, which can catalyze localized degradation. The net effect is maintenance of the PVC's physical properties, but also "chalking". In many cases, this is actually desired, particularly in white colors where chalk is not objectionable. The chalking actually helps mask the PVC natural yellowing and therefore affords increased brightness.
Chalk resistant TiO₂ products are generally encapsulated with amorphous silica and contain increased levels of alumina. The encapsulation forms a physical barrier to the transfer and formation of free radicals of electrons and holes to the PVC polymer and results in profiles and moldings with very low chalk rates.
Cristal has in-depth knowledge of the weatherability of rigid PVC products and can offer a number of solutions depending on your starting formulations and your end product requirements. For further advice on which product to use in your rigid PVC product, please contact us.