Following is and excellent article by Color Resoultions that appeared in the September/October issue of AICC's BoxScore brochure.
Slip & Slide
By: James Ford, Senior Chemist, Color Resolutions International
It’s summer-time, and while kids may be thinking of that wonderfully fun ultimate backyard water experience, many printers are worrying about the other connotations for the terms, slip and slide. Whether the contents be large appliances, or small fragile decorative objet d’art, the packaging plays an important role in the marketing and the protection of its contents.
We went to an industry expert, Ralph Young of ‘Ask Ralph’ notoriety with the AICC to inquire about our observance to an increase in inquiries to skid resistance properties of corrugated packaging. Young told us, “All containerboard and combined board properties are becoming more important to convertors, end users and retailers. Shipping is a matter of cost, sustainability and dependence on foreign sources of energy and Wal-Mart is looking at all these.” Young added, “Stack sizes are changing to cube out trucks and some are variable to accommodate maximum cubing.” “Retailers would like to see no banding and no stretch wrap, so COF becomes an issue,” added Young.
In terms of corrugated packaging, the inks and coatings have roles to play with regard to the objectives. The finished package will have specific requirements that will need to be met in order to withstand the various stresses involved with the converting, packing, shipping, and storage processes.
Two of these requirements involve the coefficient of friction (COF) and rub resistance of the package.
The COF can be thought of as a measurement of the resistance to movement of a package on a surface. A low COF would mean that a package is slick and slippery. This could lead to boxes falling off of conveyors and shifting loads on trucks. On the other hand, too high of a COF could lead to packages that are rough and potentially trouble in filling lines. So, there is always a balance between slick and unyielding that needs to be maintained. In achieving this end, it is important to realize that there are actually two types of COF measurements, static and kinetic measurements. Static COF is a measurement of the force required to move an object at rest against a surface, while kinetic COF is the measurement of the force required to keep an object in motion against a surface. In general, kinetic COF is a main concern with filling operations, while the static COF is the main concern during storage and shipping operations.
There are many methods used to measure the COF of a surface. However, these basically fall into two main categories. There is a horizontal bed method, such as is detailed in TAPPI Test Method T 816, “Coefficient of Static Friction of Corrugated and Solid Fiberboard”, which is able to quantify both the static and kinetic COF values, by measuring the resistance to movement as a weighted sample is pulled along a surface along a horizontal path. However, there is also the inclined plane method, described in TAPPI T 815, “Coefficient of Static Friction (Slide Angle) of Packaging and Packaging Materials”, which records the slide angle at which movement begins when a weighted sample is placed upon the surface. The inclined plane methods will only give the static COF, after a conversion calculation is made. The COF is calculated as the tangent of the slide angle (in radians). The relative ease of operation and less-expensive nature of the equipment, has led to prevalence in the slide angle type of measurement.
In developing inks and coatings for slide angle, formulators, such as those in our Product Development Lab at CRI, have to balance the properties of the resins and additives involved with their effects upon the slide angle and rub resistance of the coatings on the applied substrate. Hard polymers, with high Tg values, generally can provide good rub resistance, but may not provide high enough slide angles on their own. Generally, softer (lower Tg) polymers are required to get higher slide angle results. However, these tend to be tackier, and may lead to blocking, printability, and/or rub issues. Waxes or silicones are sometimes added to improve the rub resistance of inks and coatings, but these can drastically reduce the slide angle of a coating. Even a small amount of wax can cause a drop in slide angle of more than ten degrees. So, the formulator often has to balance rub resistance with slide angle. The chart below represents a common occurrence in the development of inks and coatings, namely a noticeable decrease in COF or slide angle, with an increase in rub resistance1, upon the addition of certain waxes to formulations. Simply switching out a wax in a formulation can improve the slide angle, but will often reduce the rub resistance. In a given resin formulation, it is important to balance the expectations of the customer for slide angle/COF in relation to any rub requirements. In addition, the COF of a coating may change over time due to curing of the ink film, migration of slip agents (wax bloom), or film softening (plasticizer migration). So, the age of the coating is a constant that should always be considered. While the ultimate package may display a constant COF value, many printers make decisions based upon prints taken right off the press which will have little relevance to the properties of the coatings in application. The aging effects of any coating need to be understood before basing any rejections or acceptances upon samples taken directly at the press.
Beyond the various formulation adjustments that might affect the COF of a coating, it is important to understand the various non-formulation dependent variables that may be factors in COF evaluations. The substrate plays a significant role. The porosity of the paper, the smoothness of the finish, and even the nature of the pulp itself can affect the COF of the substrate2. Inks and coatings providing a sufficient level of COF, can often begin to fail when the paper is changed. Levels of wax in a formulation for an absorbent substrate may be excessive for a less absorbent substrate, leading to a drop in COF levels as more of the wax is present at the frictional contact points. Likewise, fillers and opacifying agents that fill in voids of a porous substrate may be sources of friction if the substrate becomes less porous, causing the COF to increase. Our CRI Lab conducted a comparison using the same ink on 4 different lots of the same grade of paper, and the results are shown below. As you can see, even within the same grade of paper, significant variations can be seen in the slide angle. If you look into papers coated with silicas or waxes, you could get an even wider variation.
Another factor to consider is the relative humidity. This has been previously shown to be a significant factor in relation to COF3. So, sample conditioning will be important. Ralph Young concurs, “Printers are likely to cry out to their ink suppliers for help, but the issue is much more complex than looking at one element of the box business.” Young added, “I continue to inform our industry that there are as many as thirty different variables that affect the performance of containerboards and therefore combined corrugated board.” Adding to the inconsistencies are “recycled” substrates. Young explained, “With the increased use of recovered fibres and the variations among sorted waste, office waste, newsprint grades, DLK and OCC as fibers sources for “recycled” substrates, it can become more difficult to produce a consistent substrate. The proper cleaning of secondary fiber at the mill is critical and the quality of the finished product is dependent on the number of screening stations, separation of the fibers by length and quality, refining of the fibers, blending of the fibers and the characteristic of the paper machine.”
In conclusion, any discussion of COF specifications for corrugated packaging should include the understanding of the function of the packaging, along with the processing conditions involved in the manufacturing and shipping of the package. Process variables such as substrate variations, applied coat weight variations, humidity differences, and any possible aging phenomena need to be understood in order to reduce the variability in testing. Control of these process variables will allow for the optimization of any applied inks or coatings to the substrate.
James Ford is the Senior Chemist and Special Projects Technical Manager for Color Resolutions International (CRI) in Fairfield, Ohio. Color Resolutions International is an international packaging ink company focused on water-based and UV curing flexo inks for corrugated, flexible packaging, envelopes, folding cartons and tags and labels. For more information about CRI, visit www.colorresolutions.com or call toll free at (800) 346-8570.
Ralph Young is a Technical Advisor for the Association of Independent Corrugated Converters and is available for consultation (‘Ask Ralph’) as one of the many AICC membership benefits.
1 ASTM D 5264 – 98 “Standard Practice for Abrasion Resistance of Printed Materials by the Sutherland Rub Tester.”
2 T. Enomae et al, “Influence of Coating Properties on Paper-To-Paper Friction of Coated Paper” (Journal of Wood Science 52, no. 6) (2006): 509-513
3 P.E. Gloor, “Truth or Myth?” (Paper, Film, and Foil Converter – June 2005)
