Yield and Unit Weight
Yield is the measure of a film's coverage per unit weight. Values are expressed as in2/1b in US standard and m2/kg in metric (or SI) units.
Unit weight is the reciprocal of yield and is presented in units of lb/ream or g/m2. The film industry tends to use yield values, while the paper industry favors unit weight. Our discussion here will focus on yield, but the principles also hold true for unit weight.
Relevance and non-relevance to performance
Films are generally sold by the pound or kilogram; yet film area is what determines how many packages can be wrapped or how many labels can be produced. Therefore, yield is a critical property for determining the correct quantity of film to purchase, and it impacts the economics of the application.
OPP is a high-yield material, which makes it very economical compared to the alternatives. As show in Table 2, pound-for-pound at equal gauges, OPP offers 53% more coverage than OPET and 28% more coverage than oriented nylon.
Yield of 1 mil Film
Unit Weight of 1 mil Film
|LLDPE (linear low density polyethylene)||.92||30,100||42.7||14.4||23.4|
|HDPE (high density polyethylene)||.95||29,200||41.5||14.8||24.1|
|Hicor OHD (oriented high density polyethylene)||.96||28,900||41.0||15.0||24.4|
|OPET (oriented polyester)||1.39||19,900||28.3||21.7||35.3|
|Table 2: Yield values of common films|
What affects film yield and how well it is controlled
For a solid, uncoated film, yield is determined by resin density and average film gauge. Resin density is altered negligibly by process conditions; so yield is controlled to the degree that thickness is controlled. ExxonMobil has sophisticated on-line gauge measurement and control systems that keep machine and transverse direction thickness profiles tight to target. Because yield is such a critical property, it is also checked routinely in the lab. For all film types, film is rejected if it is outside the specification tolerance limits of ±5% of target. But most ExxonMobil film yield measurements are well within 3% of advertised yield.
Cavitated white films (OPPalyte) and coated films have additional factors that effect yield. The cavitated core of OPPalyte films reduces film density to as low as 60% of solid OPP (as low as .53 g/cc). Therefore, per unit thickness, OPPalyte films offer even higher yields than solid OPP films. Yield is the controlled property, so if cavitation density varies slightly with process conditions, thickness (not yield) will be impacted.
NOTE: For more information about thickness, review the section on gauge.
ExxonMobil coatings are applied as thin layers onto OPP base films. These coatings are denser than the base films. Therefore, gauge for gauge, yields are slightly lower for coated films.
To measure yield, a precise area of film is cut and then weighed on an analytical balance. The area is divided by the weight, expressed as in2/lbor m2/kg,and reported to three significant digits. The recommended minimum specimen size is 300 in2 (1940 cm2) and should be obtained by folding a single layer of film multiple times in the machine and transverse directions.
Example: Film is folded two times in the machine direction and two times in the transverse direction. Therefore, 16 layers of film will be cut at once. The folded film is precision cut with a 5 inch (.127 m) diameter round die, and the measured weight is 3.496 grams. Based on this example, the following equations demonstrate the yield and unit weight calculations in US standard and metric units.
Yield (in2/lb) = Total specimen area = π(2.5)2 in2 x 16 x 453.59 g/lb = 40,761 in2/lb Weight of specimen 3.496 g
It is reported as 40,800 in2/lb.
Yield (m2/kg) = Total specimen area = π(.0635)2 m2 x 16 x 1000 g/kg = 57.976 m2/kg Weight of specimen 3.496 g
It is reported as 58 m2/kg.
Unit weight (lb/ream) = 1 = 1 lb x 432,000 in2 = 10.598 ~ 10.6 lb/ream Yield 40,761 in2 ream
Unit weight (g/m2) = 1 = 1 kg x 1000 g = 17.249 ~ 10.3 g/m2 Yield 57.976 m2 kg
CAUTION: Although yield is related to thickness, it is inaccurate to use a micrometer to assess yield. Micrometers lack the necessary accuracy and precision, and they only measure thickness at a small point, whereas the previously described method directly measures average yield over a large area. Also, thickness cannot be converted to yield without knowing the exact density of the film.
Basis weight: Basis weight is a common paper industry term for unit weight, or weight per unit area. Units are lb/ream. "Ten pounds of poly" means 10 lb/ream, which equals 43,200 in2/lb (because a ream equals 432,000 in2) or .70 mils of LDPE (at a specific gravity of .92).
MSI: MSI stands for thousand square inches. Yield can be converted from in2/lb to MSI/lb by dividing by 1000. Film pricing is commonly quoted in $/lb or $/MSI, and yield can be used to convert from one set of units to the other.
Ream: Ream is a paper industry term equivalent to a coverage area of 3000 ft2 or 432,000 in2.