Conditioning
This chapter describes procedures used to confirm that test samples represent common recent experience with environmental conditions that might affect the values obtained when they are tested. Temperature and relative humidity are the most common variables to control to provide such assurance. For the purpose of simply eliminating environmental effects as a source of measurement error, a set of standard temperature and relative humidity conditions apply for most flexible packaging materials. The time needed to equilibrate to stand conditions may vary with material. For those applications that are dependent on environmental conditions for their functionality (e.g., antistatic or antifog additives) materials must be conditioned at those conditions before making measurements.
Keywords
Antifog; ASTM; electrostatic charge; humidity sensitivity; ISO; measurement; sample; special conditioning; standard conditioning; temperature sensitivity
The basic purpose of all measures is to obtain information about a “sample” and to use that information for deciding to do (or not do) something. One measurement (a “datum” typically presents very little information, but considered in the context of other similar measurements (“data”), the information supports a decision. “Similar” here includes many important relationships including:
1. Is a sample representative of what the decision will affect?
2. Does the measure use techniques and devices common to other data?
3. Have environmental factors influenced the value of this measure different than value of other data?
Conditioning consists of efforts to control environmental factors that can influence the value of a measure. Temperature and humidity represent the usual factors requiring control, but they interact with others, such as time and pressure, that must be standardized for some measures as well. Measuring physical and chemical changes over time (e.g., migration through films and cross-linking reactions) in particular demand precise control of environmental factors. Standard conditioning environments are used if representative measures of materials are sought. Standard conditioning environments provide information about how materials—and their measures—can change if their use environments change.
Standard Conditioning
“Standard conditioning and testing atmospheres for paper, board, pulp handsheets, and related products” (Test Method TAPPI/ANSI T 402 sp-13) details the standard paper industry conditioning environment for temperature and humidity.1 It also specifies procedures for handling these materials so that they may reach equilibrium with the respective atmosphere. This standard practice does not include special conditioning and testing atmospheres, such as those that attempt to simulate tropical or arctic environments.
The “conditioning” environment of the TAPPI Standard requires 50%±2% RH and 23.0±1.0°C (73.4±1.8°F). Webs of and containing paper at 50% RH can have different physical properties depending on whether the sample was stabilized at 50% from higher or lower relative humidities, For this reason, a standard “preconditioning” environment of 10–35% RH and 22–40°C (72–104°F).
ISO 291: 2008 provides conditioning environments for plastics [1]… the conditions specified here are same as the TAPPI standard. Special atmospheres applicable to a particular test or material or simulating particular climatic environments are not presently included in this International Standard, but the 1997 version suggested 65% RH and 27°C (80.6°F) for tropical country use. Both temperature and humidity can influence plastics’ properties. Properties of polyolefins themselves do not vary greatly with humidity, although copolymer polyolefins, and condensation polymers may (Chapters 23–25). As thermoplastics, ambient temperature variation can significantly influence polymer properties.
Table 30.1 shows the dependence on relative humidity of the equilibrium water content of Nylon 6 [2]. Conditioning of nylon-containing samples with standard relative humidity conditions is critical for producing comparable physical testing results.2
Table 30.1
Influence of Relative Humidity (RH%) on Equilibrium Water Absorption (in %, at 23°C-Air) of Nylon 6
| Type | 30% RH | 50% RH | 62% RH | 100% RH |
| Nylon 6 | 1.1 | 2.75 | 3.85 | 9.5 |
The industry around the world has standardized and 50% RH and 23°C for its testing environments and simply refers to these as “TAPPI Conditions.”
Special Conditioning
In particular contexts, the effect of temperature and humidity on flexible packaging materials may be of interest themselves. For example, the surface of films made from “soft” resins (e.g., high copolymer content EVA) will deform and conform to adjacent surfaces under pressure. This can lead to interlayer adhesion between layers of a roll of material (called “blocking”), particularly if high temperatures soften the plastics even further. Rather than test whole rolls of film wound with different tensions, individual sheets of film can be stacked on one another and compressed with known weights.3 Conditioning of the test stack in elevated temperatures and humidities (e.g., 37°C–100°F and 90% RH) provides insight into the effects of storage conditions on film blocking.
At the other extreme, cold temperature can embrittle plastic films making them much less durable than at 73°F. Conditioning films at temperatures they can expect in their use situations allow quantification of the effect.
Additional special conditioning environments have been adapted to assist in measuring the performance of flexible packaging.
• Packaging films that discharge accumulated electrostatic charge before this force can become high enough to damage electronic products within usually required adsorption of moisture onto their surface to be effective. To measure surface resistivity of these products, the US military requires conditioning samples at 12±3% RH and 73±5°F for at least 48 h prior to testing in the same environment [3].
• Films packaging moist products stored in refrigerated conditions will appear hazy in use because of moisture from the products condenses in small droplets on the inside of the film, unless a special “antifog” additive lowers the surface energy of the film so much that the droplets coalesce into a uniform thin layer. To assess this effect, film is conditioned at 4°C (39°F) in low humidity before it is exposed to cold water at this temperature.
Special conditioning environments are appropriate when a measure attempts to model the behavior of flexible packaging materials when used in such conditions. Standard conditions are appropriate for general measurement of material behavior under average conditions.