Water Vapour Barrier Coatings for Paper – Measuring their Effectiveness

Introduction

When most people in the paper industry think about paper coatings, they typically think about coatings based on inorganic clay-like materials as a filler, which improve aspects of the paper such as smoothness, porosity and brightness. This type of coating constitutes by far the largest majority of coatings applied, especially in paper mills. The remainder of the coatings market consists of specialised coatings that provide some enhanced performance characteristic. One of the more important and challenging requirements of a paper coating is where control over the flow of water vapour is required. Water vapour affects paper in almost every application – from print quality and paper jamming, through to packaging strength and suitability.  In some applications permeability must be high, in others low, in yet others in-between.  Permeability is the critical factor in or most applications. In this article we consider water vapour barrier coatings and how their effectiveness is measured.

Water Resistant Barriers vs. Water Vapour Barriers

1. Water Resistance - Without its sensitivity to water, we would not be able to make paper, certainly not in the way we do. However, especially when used in packaging, paper products need some resistance to liquid water. The lowest grade of improvement is water resistance. This is commonly done with polymeric coatings, in some cases enhanced with fillers and/or wax emulsions. Success at achieving water resistance is typically measured by use of water absorption methods such as the “Cobbs” test. While water beading (the “water off a duck’s back” effect) is often related to water resistance, there are now many waxless  coatings that remain wettable  by water while still maintaining excellent water resistance. For many packaging applications, especially in secondary packaging, good water resistance provides adequate protection for the contents of the packaging.
   
2. Water Vapour– although products that are water resistant are often though to be water vapour resistant and vice versa, this is not always the case. Returning to the duck, although the water resistance is sufficient to keep the duck afloat, there is very little difficulty in blowing through its feathers, and water vapour flows as freely as does air. The property of the feathers that influences their surface tension has very little influence on their permeability to water vapour. Likewise there are coatings that give good resistance to liquid water, yet provide little or no barrier to water vapour. Conversely there are water-soluble coatings and films that provide a vapour barrier, but not any liquid water barrier. Of course there are also coatings that provide a good barrier to water in either phase.

Measuring Water Vapour Barrier

For many years, there have been techniques to measure water vapour permeability. Known as WVTR (water vapour transmission rate) or MVTR (moisture vapour transmission rate) there are  two main techniques, each with their own advantages and disadvantages.

1. Gravimetric – this technique is based on the change in weight caused by the passing water vapour through the material being tested. There is a series of local and international standards for these tests, which, depend on the material being tested. In the paper coatings regime, a period of conditioning is specified. A cup of water absorbing material is covered with the sample and sealed to the cup.  The cup is accurately weighed and placed in an humidity and temperature controlled cabinet. The cup is reweighed daily. In some cases the test is continued until equilibrium is reached, but in others the test is continued for a fixed period or until no more water can be absorbed. There are two common standards for the temperature and humidity conditions:

 

	Temperate Conditions	Tropical Conditions
Temperature °C (F)	25 (70)	38 (90)
Relative Humidity %	75	90

1. WVTR Meters – as these are proprietary in their designs, they vary considerably in appearance, mechanism, components and even in measurement sensor. However, the leading instruments all work on a similar principle. One side of the sample is exposed to humid environment, and the water vapour that passes through is swept to a sensor by an inert gas. One example of this type of instruments is manufactured us in the UK, it allows the sample to be fixed between two halves of a stainless steel chamber and is housed in an accurate thermostatically controlled oven. In the lower half of the chamber, the desired humidity conditions are generated, and dry Nitrogen gas is passed through the upper half, . All of the water vapour passing through the sample is swept to the detector,  and an accurate measurement is made. In most cases when dealing with water based coatings, equilibrium is reached with a few hours, although in some cases, results are obtained in as little as 30 minutes.  This compares to days, weeks and even months for gravimetric measurements of the same materials.

Benefits and Disadvantages of the Techniques

Speed – WVTR Meters are significantly faster, and permit the timely quality control of coated stock. This can reduce waste in paper, coatings and machine time if the end product drifts out of spec.  Typically an hour with a WVTR meter compares to 3 or 4 days of gravimetric testing.

Accuracy and Reproducibility– Even when the gravimetric technique is done perfectly, there are issues relating to cup design and leakage, desiccant preparation and testing, and weighing errors that cause inaccuracy and poor reproducibility. Instrumental techniques tend to reduce inaccuracies and increase reproducibility.

Standards – Both techniques have standards that impact their acceptability; however, more (and a more confusing plethora of) standards are based on gravimetric techniques.

Cost  - although a good humidity cabinet will cost almost as much as a basic instrument for measuring WVTR, the most expensive WVTR meters are more  costly. As relative humidity and the diffusion process are significantly sensitive to temperature, the use of normal laboratory ovens for these tests is to be discouraged.

 Sensitivity – as the chemistry and technology of barrier coatings improves, the ability of instrumental techniques to measure very low rates accurately becomes more significant. Instruments such as the Versaperm WVTR meter for example, are routinely used to measure WVTR on aluminium foil laminations.

Correlation between the Techniques

Studies comparing the correlation between the gravimetric and instrumental techniques have found that in the case of paper coatings, the gravimetric results tend to be higher by an average of 30-50%. However, results from different laboratories conducting the gravimetric tests have varied by up to 50%, whereas results between different instrumental techniques vary by less than 20%. Instrument manufacturers find themselves in the curious position of deciding whether or not to try to match a method that they believe to be intrinsically less accurate.

Conclusions

While there is little doubt that many laboratories will continue to measure WVTR by gravimetric methods, for many others the switch to instrumental techniques is offering considerable advantages. These include faster QC control and the cost savings involved, greater accuracy and reproducibility, and an ability to read very low WVTR measurements.