PLASTIC FILM MANUFACTURERS PROCESS ASSESSMENTS

This protocol is a tool for setting-up and running trials designed to incorporate post-consumer High-Density Polyethylene (HDPE) resins into blown film products. This tool includes the following sections:

• Trial set-up
• Start-up
• Process run
• Forms to collect trial conditions - "Blown Film Process Audit" and testing results - "Plastic Film Testing"

The protocol also includes background information on the following:

• Blown film process
• Using post-consumer HDPE resin
• Property testing requirements

Because each film production operation is different, the processing variables and testing procedures should utilize company practices when available. When trials are run to incorporate a recycled material for the first time or to increase the recycled co ntent of the film, it is important to have established baseline film processing parameters and information on the material property characteristics for standard products. This baseline information will be compared to data collected during the recycled ma terial trials.

Blown Film Process

In the blown film production process, plastic melt enters a die, flows around a mandrel, and emerges through a ring-shaped opening in the form of' a tube. Air is introduced into the tube causing it to expand and form a bubble. An air flow around the out side of the bubble cools and solidifies the melt. The air is contained in the bubble by the die at one end and by the nip rolls at the other end. An even pressure of air is maintained to ensure uniform thickness of the film bubble.

Both stretching the bubble radially, and pulling it away from the die axially orient the plastic, improving its strength and properties. After solidification, the film bubble moves into a set of pinch rolls where it is flattened and rolled onto a winder. Some extrusion lines have printing equipment and bag-making machines on-line. Simpler extrusion lines form the film or sheet and then perform the printing and bag-making functions off-line.

Coextrusion is a variation of this process. The die is designed with multiple flow channels so that multiple layers may be formed. Multiple layers are typically used in order to protect the product from moisture, air, etc. The number of layers can rang e from two to as many as ten layers in complex systems.

The typical range of film thickness is .0001-.050 inch. Common products formed by the blown film process include garbage bags, can liners, agricultural films, grocery bags, and thin films for paper and tissue products. Common materials used include poly ethylenes, polypropylenes, ethylvinyl acetate (EVA), and flexible polyvinyl chloride (PVC).

Reference: Berins, Michael, Plastics Engineering Handbook of the Society of Plastics Industry, p. 102, 1991.

Using Post-Consumer HDPE

The information associated with this protocol relates to using post-consumer High Density Polyethylene (HDPE) from milk bottles. This material is the most widely used post-consumer resin (PCR) plastic in blown film operations. Understanding the end prod uct requirements, the effect of PCR HDPE on film properties, and the effects of different blends of materials on film properties will help determine which blends to use and whether the use of PCR HDPE will meet customer requirements.

The goal of the process trials will be to make a product that meets or exceeds all customer requirements. It is difficult to predict how different material blends will interact with changes in materials and changes in process variables. The new blends m ay make some end-use properties better and some worse. Thus, it is important to test for all product requirements to ensure a quality film. The performance of trial products compared to standard company products and standard materials processability can determine if acceptable quality is achieved during the different trials. Note: If the end product is an FDA regulated product, the use of PCR materials may be restricted.

According to Joseph Ogando, editor of Plastics Technology, there are three key issues when using PCR in film applications: good mixing, selection of virgin blend materials, and strict process controls.

Material Mix. Obtaining a homogeneous material mix is important when using a blend of PCR materials. The two main issues related to mixing are blending of the raw materials prior to introduction into the extruder, and melt mixing within the extru der. Proper blending of materials prior to introduction into the extruder may be a relatively simple process due to the development of continuous flow blending equipment. However, less complex manufacturing operations that process in batches may require closer scrutiny when preparing materials.

Melt mixing within the extruder is the key to obtaining a good PCR/virgin blend. PCR from milk bottles have a higher melting point and viscosity when compared to other polyethylenes. Because milk bottle resins were not designed for blown films, their me lt characteristics are much different than standard blown film resins, such as low density polyethylene (LDPE) or linear low density polyethylene (LLDPE).

Low shear and controlling melt temperatures are important factors in obtaining a homogenous material mix and in preventing degradation of the resin. Because PCR has already undergone multiple heat histories, preventing excessive heating is important. A plastic material gains a heat history each time it is heated, melted, and formed into its final state.

There are two approaches to obtaining low-temperature, low-shear mixing. The first involves the use of longer extruders in the 30:1 L/D range. The longer extruders mix blends of materials better and have the flexibility to process any material. The oth er approach advocates the use of shorter extruders, 20:1 L/D in order to decrease the residence time of the material. Many equipment suppliers also recommend the use of barrier screws. Barrier screws are designed to accommodate blends of multiple materi als.

Selection of Virgin Blend Materials. Selection of virgin blend materials is another important consideration when using PCR materials. PCR materials generally have higher densities than standard blown film resins. Consequently, the density of PCR blends is higher than those of virgin blown film resins and poor impact and tear properties may result. Lower density virgin material can be used to bring down the overall blend density.

Dow, Inc. recommends the use of ultra-low density polyethylenes (ULDPE) in order bring down the overall density of PCR content blends. OxyChem Inc. recommends the use of higher-molecular weight HDPE for applications with PCR levels above 25% because it i mproves tear strength properties. Mobil Polymers advocates the use of its hexene-linear low density polyethylene (LLDPE) for accommodating PCR without loss of film strength. Note: Attachment A describes HDPE PCR effects on film properties.

It is also important to choose the optimum level of PCR. At certain ratios, using PCR can actually improve end-use properties. Processors should experiment with a range of PCR levels to obtain the optimum end-use properties

Strict Process Controls. Strict process and quality controls are the third consideration when using PCR materials. Continuous screen changers have been increasingly used in blown film applications because PCR materials may have higher contaminati on levels when compared to virgin materials. PCR inconsistencies such as contamination and melt flow variation can be addressed by the use of specialized equipment. Gravimetric yield controls and gear pumps can be used in order to accommodate the incons istencies of PCR materials. The lower melt strengths of PCR blends can cause unstable bubbles and fluctuating neck heights. Chilled air cooling systems or redesigning the air-flow patterns are two techniques that can compensate for this characteristic.

Reference: Ogando, Joseph. Plastics Technology, "Blowing Film With PCR Is Not That Tough", p. 24, December 1994.

Trial Set-Up The purpose of the trial set-up is to make sure all the equipment, materials, blend formulas, initial trial conditions and operators are prepared. By being prepared, the trial will run smoother, take less time, and the information gathered will help iden tify the best combination of materials and processing conditions to produce the desired film properties. The initial trial conditions can be set by determining the blend of materials and the initial machine conditions. As part of the process run, you ma y plan to run a number of different blends of materials. Be prepared to adjust the blends after the first or second trial based on test results.

It is important to understand the end product application. The goals of these trials are to use different blends of materials and to vary machine conditions to optimize the desired end product characteristics. During trial set-up, determine the standard product that will be used for comparison with processing and testing characteristics generated during the trials.

The attached forms will help organize processing and test information for trial data comparison and to help determine other trial conditions. The following is an example of typical trial set-up procedures.

1. Identify materials to be used. The information for the "Blown Film Process Audit" form should include material description, vendor, lot number, any material tests supplied by the vendor, and any comments. Some of the material characteristics th e vendor may supply are melt index, density, and gel count. If material must be ordered, make sure the lead time fits the time frame for the trial.

2. Collect information from the operators to estimate the time required to run each trial. The information should include times for cleaning and filling the hopper, equipment warm-up, start-up of the film line, purge time to remove any material in the ex truder, and time for the line to stabilize. Note: Estimate conservatively to ensure sufficient time for all of these conditions. This information will determine the practical number of trials given the available time and resources.

3. Determine the blend of materials to be used for each trial. The number of blends will determine the number of trials. Starting with the minimum targeted recycled content level, increase the percentage of' recycled material in the blend by 10% or 20 % depending on the number of trials. Determine approximate starting machine conditions, such as temperatures, pressure, amperage, and air flow. Assign trial numbers.

4. Make sure you have the "Blown Film Process Audit" forms, "Plastic Film Testing" forms, marking pens or flags to identify sections of film to be tested, and a tape measure.

5. Make sure all operators and test personnel have a good understanding of the trial and their responsibilities

6 Plan to clean feed hopper and transport equipment during start-up to minimize purge time.

7. Record the equipment information on the "Blown Film Process Audit" form

8. Record baseline processing conditions and film test characteristics for standard products so this information can be used for comparison to the PCR trials.

If after Trial 1 and 2 have been run, the tear strength of Trial 1 is marginal and Trial 2 is unacceptable, do not continue to run higher recycled contents with the current virgin blend, because the tear strength will continue to decrease.

You may then decide to try a new Trial 3 with 50% recycled, 40% virgin-HDPE, and 10% virgin PE of a lower density in order to increase tear strength. (Note: incorporating the use of a lower density PE will bring down the overall blend density. This will s erve to improve overall performance properties.) If performance properties are still insufficient, you may attempt to increase the percentage of the lower density PE.

Start-Up

The typical start-up operations include the following steps:

• Material preparation,
• Line start-up,
• Purging of any material left in the extruder, and
• Line stabilization.

During the start-up of each different blend, be sure to purge the extruder completely and let temperatures, pressures, amperage, and other machine conditions stabilize so that representative samples for testing comparisons can be obtained.

During start-up, allow for warm-up time and stabilization before any test samples are identified. Note and record any differences compared to standard processing. The temperature settings and other machine variables may have to be adjusted several times to obtain acceptable processing conditions. The following is an example of a typical start-up operation:

Material Preparation Steps

1. Weigh each material as accurately as the equipment allows, record weights, calculate percentage of' each material, note any comments.

2. Mix materials so the blend of materials fed to the extruder hopper is homogeneous. Record time and any comments

3. Make sure hopper and feeding equipment is clean, then load hopper.

Line Start-up Steps

1. Using the start-up conditions determined during the set-up, start up the film line.

2. The temperatures and extruder RPM may need to be adjusted to achieve the desired extruder amperage, extruder pressure, and throughput.

3. Purge all material left in the extruder. This will stabilize the line and test results will reflect actual trial blends.

4. Once the line is running and the equipment has stabilized, take all machine measurements and measure film thickness.

 

The process run portion of the trial is when machine conditions will be recorded, machine variables will be adjusted to determine their effects on film properties, and samples will be identified for testing.

During the process run, note process line conditions on a regular basis (every 10 to 15 minutes) even if no variables have been adjusted. The machine operator's comments on how the trial runs compare to standard products may be valuable in determining op timum machine conditions or material blends. Flagging or marking the film to coincide with changes in process variables will help to identify where process changes were made.

In order to process new blends of materials you may need to adjust standard processing conditions. Trials using different blends will be used to determine the blend effects on performance properties.

For example, Blend A may increase tear strength but decrease tensile strength. Blend B may have no effect on tear strength but increase tensile strength. When using different material variations, machine conditions may not have the same effects on film properties. For example, raising the frostline height for Blend A may increase tear strength. For Blend B, lowering the frostline height may increase tear strength. The following is an example of a typical process-run operation:

 

1. Adjust temperatures and air flow to stabilize the film line. Record any machine line changes and record machine variables throughout the trial. Measure the film thickness every 5 - 10 minutes. Record machine conditions.

2. If multiple thicknesses are to be run, increase or decrease line speed for each new thickness. Record all machine conditions and re-measure thickness.

3. The frostline height may have an effect on strength. Compensate for fluctuating frostline heights by adjusting temperatures, throughput, or airflow.

4. Mark the roll (using a paper flag or marking pen) when changing process variables or pull samples for each film section for testing; note on log sheet so machine conditions can be compared to test results.

5. Note film smoothness, frostline height and stability, printability, and any other comments by operators on how the test section compares to standard processing.

Property Testing

Property testing is important for a number of reasons. These may include: to meet customer standards and specifications; to provide quality control; to verify the manufacturing process, and to establish a history for new materials. Product testing of bag s and films is used to determine variances in film characteristics due to the addition of PCR materials. The material blend or machine conditions may then be modified to meet standard product requirements. Often times, PCR blends meet or exceed customer requirements with little modification to existing equipment.

Testing often includes material evaluation such as density, and mechanical property evaluation such as tensile strength. The methods of testing often vary depending on the capabilities of the manufacturer. For example, testing the tensile properties of' bags may entail filling the bag with a weight rather than using the traditional tensile testing machine per ASTM specifications.

The specific material and mechanical property tests will vary depending upon performance requirements and customer specifications. The following are tests that may be used when evaluating recycled resins and PCR content plastic films or bags:

Density. A materials characteristic, the specific density of the overall blend has an effect on end product properties. Lower density blends often have better mechanical properties in film applications. Density is determined by ASTM specificatio n D792 or D1505.

Melt Index. A materials characteristic, the melt index of the blend may affect the processing and melt mixing characteristics of the blend. Blends with lower melt indices may decrease throughput and require increased mixing in order to obtain con sistent mechanical properties.

Gel Count. A materials characteristic, gels are materials composed of oxidized or high molecular weight materials. The presence of gels in plastic films is objectionable due to appearance and to the problems associated with printing on the films. Gel count is determined by ASTM specification D-335 1-74.

Tensile Strength. A mechanical property, tensile strength is a measure of the maximum stress a material will withstand when subjected to a load in tension. ASTM specifications for plastic films include D 88291. A simpler method is to load the fi lm or bag with a weight.

Tear/Shear Strength. A mechanical property, tear strength is a measure of the maximum stress a material will withstand when subjected to a load in shear. ASTM specification for plastic films is D 1004-90.

Dart Drop Impact Strength. A mechanical property, dart drop impact measures the toughness of the material by introducing a polyaxial load. ASTM specification for plastic films is D 1709-91.

Haze. An optical characteristic, haze is a measure of the clarity or transparency of the material. ASTM specifications for plastic films is D 1746-92 and ASTM D l003.

Gloss. An optical characteristic, gloss is a measure of the reflectance of the material. ASTM specification for plastic films is D 2457-90.

In addition, there may be a number of other material or mechanical properties that are important to the manufacturer. These will vary depending on the performance requirements and customer specifications. Other characteristics that may be tested include : burst/seal strength, odor, freeze resistance, print quality, and modulus. Attachment A describes the HDPE PCR effects on film properties.