Two factors are needed to make the modern cotton packaging, storage and transport system effective: a well-formed module that resists moisture collection and a cover that prevents both moisture penetration and wind loss. These principles apply for both conventional modules and modules formed by the on-board modeling harvesters. For on-board harvester modules, the shape of the module is controlled by the machine, but the producer/operator must still be concerned with the integrity of the protective cover.
Module Tarp Construction
Top Surface Materials
Covers for conventional modules are manufactured from a variety of materials that resist water penetration. All modern covers are made of a plastic material (typically polyethylene), either as a film or woven from tapes of differing sizes. The woven fabrics may be coated on either one or both sides, and the coatings can be polyethylene or polyvinyl chloride.
All module covers should have UV-inhibiting chemicals incorporated into the cover material. While the type and quantity are often not disclosed to buyers, butylated hydroxytoluene (BHT) stabilizers should be avoided because they tend to turn seed cotton yellow when they come in contact with the cotton. High levels of UV-light stabilizers are recommended for all regions of the Cotton Belt. The three primary types of module cover materials available today are:
- woven polyethylene with double coated polyethylene laminate (usually called “poly” covers),
- polyethylene terephthalate (PET) scrims with double-coated polyvinyl chloride film laminate (usually called “vinyl” covers) and
- polyethylene film.
Top Surface Factors
The top surface can be formed in various ways, depending on the manufacturer. Most covers manufactured today do not have seams in the top surface, because of the potential for leaks. Any seams in the top surface of laminated, coated fabrics should be heat sealed to minimize leakage. Manufacturers often offer a range of colors for the top surface. Color does not make a difference in performance in most regions of the Cotton Belt. The exception would be south Texas, where the more intense solar radiation in the summer harvest period would encourage the use of lighter colored tops.
Plastic mesh netting is sometimes used instead of module covers to limit wind losses in the more arid cotton producing areas, as it is assumed that the modules will not be exposed to rain. The netting primarily serves to prevent loss of seed cotton during periods of high winds, but provides no protection from water. Most gin managers prefer the mesh netting not to be used, as the modules are exposed to possible moisture damage. The netting is also difficult to remove at the module feeder and presents a disposal problem for the gin.
Skirts (the vertical portion of the cover extending down the side of the module) are available in a variety of forms. Module skirts can be fitted or open-ended. The material on the ends of the cover is typically the same as the top surface. The skirts can be made of coated fabric (typically of a lighter weight than the top) or a mesh fabric. The mesh is preferred in regions where seed cotton moisture and cool morning air can cause condensation on the underside of the cover. With the mesh sides, air flow is enhanced across the module top to dry and remove the moisture. Manufacturers offer the side skirts in both straight and V shapes. The V shape skirt is considered by some users to be better than the straight sided skirt because they tend to stay on the modules better during high wind conditions.
Belts and Tie-Down Loops
Module covers include a method for firmly attaching the cover to the module. Ropes and belts are the common methods of holding module covers on the modules. They are enclosed in sleeves sewn around the bottom of the cover skirt that allows the rope or belt to slip as they are pulled tight. Adequate tension on the rope or belt is necessary to keep the cover on the module during windy conditions.
Some cover designs have two tensioning points: at the bottom of the skirt and midway up the skirt. Flat belts normally utilize buckles for maintaining tension (Figure 19). Ropes are provided with rings that can be used to tie off the tension. It is important that the knots for ropes and buckles for belts are easily untied and unbuckled and do not need to be cut for removal. Use only slip knots when tying ropes. When the module is exposed to wind, the cover will raise off the module, tightening the belt or rope, which will cause any knots to become extremely tight and compressed. In this situation, gin workers will typically cut the belt or rope in the interest of saving time. Properly tied knots and buckled belts will prevent rope and belt repairs that might hinder slippage through the sleeves and undesirable knots or ties not easily untied or unbuckled at the gin. The extra length of rope or belt remaining after tying or belting should be placed inside the module cover to prevent entangling in the module truck or module feeder.
High Wind Situations
Figure 20 shows a tie-down under the module. Placing module tape (not baling twine or similar material!) under the modules is the recommended method. Driving a tie-down tape through the module requires a great deal of force and has the potential for both personal injury and placing contaminants into the seed cotton.
All module covers should have labels that indicate the manufacturer, address and phone number, model name or number, the date the cover was manufactured and other pertinent information. An intact label is critical for proper management of an inventory of module covers, as managers commonly use years in service to determine when a cover should be discarded. Never intentionally remove labels from covers.
Cover Life and Management
Module covers have a limited life, and should not be used beyond that period. That life is limited by the damage that occurs to the cover, compromising its ability to resist water penetration. Damage can occur from multiple factors, and understanding the impact of the damage sources can aid in the management of an inventory of covers. As was shown earlier, even covers with moderate use can result in seed cotton moisture levels high enough to lose significant value.
The most severe and obvious damage is often caused by workers handling, installing or removing the covers. Proper practices can avoid this damage. Following are practices that can minimize man-made damage.
- Refrain from excessive dragging and in particular dragging over areas with sharp and pointed objects.
- When hauling covers to the field, pay attention to where covers are placed. Always place covers on a clean, flat surface, free of sharp pointed objects.
- Refrain from walking on covers, either on a module or on the ground.
- Belts and ropes for tying down covers have a tendency to become abused during use. If the buckle or D rings fail, set the cover aside for repair and use another. Tying a hard knot in a belt will usually result in having to cut the belt at the gin, requiring a more expensive repair.
- After use, fold and roll the covers properly for easy handling and reuse on the next module.
- Store covers in dry enclosed areas free of rodents.
- Module covers should not be stored in a muddy condition.
Wind can have a significant effect on module covers in two ways. At lower speeds, wind causes the cover fabric to flap. Flapping causes the cover to bend and flex, resulting in a breakdown of the protective coating similar to that caused by exposure to UV rays. In fact, it is often not possible to distinguish which of these sources is the cause of cover damage. Since weather exposure includes both solar radiation and wind, both contribute to the gradual breakdown of the coating. When the wind is very high, the damage can be dramatic and catastrophic, such as when the base fabric of the cover rips along a seam or other stress points (Figure 22).
Module exposure to wind varies greatly across the Cotton Belt. Wind concerns are not great in California and Arizona, but significant wind damage often occurs in the Texas High Plains and Oklahoma. Severe weather in the High Plains is often related to passing weather fronts that generate wind speeds greater than 50 mph. Such high winds cause intense strains on the module cover, which results in torn covers and broken tie-down straps. Complete removal of the tarp can result in seed cotton blowing off the module and wetting of the seed cotton in the module if rain also occurs.
The Texas Gulf Coast, Mid-South and the Southeast can be subject to long periods of high winds during hurricanes and tropical storms. These areas are also subject to long periods of rainfall that have resulted in significantly increased moisture in the module. It is best to build modules with the length of module parallel to the predominant wind direction.
Cost and Performance
Covers vary in price from approximately $65 to $125 each. As with many products, higher performing covers generally have a higher price. However, the cost of even the highest price cover is less than the lint value that was lost due to poor covers in the 2004 High Plains storm event. Vinyl covers generally have the highest cost, but perform consistently well in a wide range of conditions. They have the disadvantage of being significantly heavier than the other cover types. Film covers are more moderately priced, lighter weight and most suitable for regions where high winds are not expected but significant rainfall occurs. Coated woven covers have the widest range of prices and performance. The better performing models are suitable for all regions of the Cotton Belt, but purchasers should ask for performance histories of the models when making purchasing decisions. No performance standard exists today that can be used to judge the relative performance of available covers. Information for making purchase decisions is available from the cover manufacturers.
Inspection and Repair of Covers
Inspection of covers should occur both in-season and at the end of the ginning season. Typically, covers are used on three to five modules per season, and each time a cover is removed from a module at the feeder, an inspection should occur. Typical practice is to lay the cover out on the floor near the feeder, fold it over to 1/3 of the width and roll it up using a powered or manual roller. Train feeder employees to observe the cover and look for larger defects such as rips, failed seams, or holes in the top surface and an inoperable belt or rope. If such defects exist, that cover should be placed in a “Do not use” pile to prevent that cover from going out onto another module. Those covers can be inspected for possible repair at the end of the season. Inspecting for pinholes and coating degradation is not feasible during the season because of the time required for an adequate assessment.
End of Season Inspection
Following the end of the ginning season, inspect the covers more thoroughly to determine if they are suitable for use in the next season. One simple means of determining which covers should be removed from the inventory is to use age as a determining factor. The manufacturing date shown on the label can be used to identify covers too old (a common age limit is three years) to be used again. Inspection and repair can be done by gin employees or by companies that specialize in cover inspection and repair. In addition to the larger defects that can be observed, the cover should be placed on a rack or light table that will allow light to pass through any pinholes in the coating.