The percentage of regrind plastic mixed with virgin plastic significantly affects the performance of plastic parts in electrical and electronic equipment.   Underwriters Laboratories Inc. has worked with the industry to develop the criteria and test methodology to ensure the safe use of regrind.

What is regrind?

Regrind materials include ground or chopped flash, runners, sprues and non-contaminated rejected parts that are produced in-house by the molder in initial molding processes.   These materials are crushed to smaller size and recycled with virgin materials.   Regrind excludes materials that are chemically reconstituted or repolymerized, or regrind bought in bulk and used by another company. It only applies to post-industrial (pre-consumer) waste from the same manufacturer.

What are the limitations for using regrind?

In 1980, with input from the industry, UL established the current limit on the use of regrind materials under the Standard for Safety of Polymeric Materials - Fabricated Parts, UL 746D. It was accepted that the use of thermoplastic regrind at levels less than 25% by weight (0% for thermosets) does not significantly compromise the properties of the virgin material and does not require further investigation. The use of thermoplastic regrind at levels exceeding 25% by weight requires a separate investigation to verify that the material meets minimum levels of performance with regard to critical properties.  

What adverse effects does increasing regrind proportion have on plastic properties?

Severe deterioration of the mechanical performance of the resultant part is expected from increased regrind percentage used with virgin materials.

Figure 1 shows the effect of increasing proportion of regrind thermoplastic material to virgin material. After a threshold, the properties of the resulting material begin to degrade.   This degradation can cause hazards in the use of the plastic part that is installed in equipment.   Therefore, for applications where mechanical properties are important, the margin of safety decreases as the proportion of regrind to virgin materials increases.

Figure 1

Plastics are becoming more commonly used for applications that involve

• Mechanical loading - Engineering applications
• Direct contact with current carrying conductors
• Exposure to ultraviolet (UV) light, moisture, chemicals, heat, etc.

For these applications, properties such as strength, impact resistance, softening temperature, optical clarity, insulation, and outdoor weatherability are very important. The molecular weight of certain materials is an important factor for many of these properties. Regrind material tend to result in lower molecular weight than virgin material mainly due to chain scission brought by the multiple thermal exposures of the mold history. Therefore, the overall average molecular weight of the mixture of virgin and regrind will be less than that of 100% virgin material; as a result, too much regrind will negatively affect key properties of the material.

An example of how regrind percentage can affect performance of plastic parts - Wire & Cable

To illustrate the above, consider the application of plastics as an insulating material for current-carrying conductors—wires and cables.

The property requirements for the plastic for this application are

• Good mechanical strength to support the conductor
• Good insulation properties
• Good tracking resistance

The above properties for regrind material will be lower than those of pure virgin material due to lower molecular weight. The insulation given on the conductor with regrind material will fail at lower levels when compared to insulation comprised of 100% virgin material.

Figure 2 shows the dependence of Mechanical Properties on Molecular Weight. The material with lower molecular weight will lose strength very fast and may eventually crack or craze under severe conditions. Such damage to the insulation exposes the conductor and could in turn cause fire, shock, or injury hazards.

Figure 2

In additional, degradation of the properties may occur if the plastics are fiber-reinforced and repeatedly blended with virgin material. Long fiberglass, used to provide reinforcement, is mechanically broken into shorter segments after each regrind cycle, resulting in a reduction in the strength of the final product.    This may reduce the performance of the plastic part to an unacceptable level.

What is UL's recommendation on the use of regrind?

No regrind is accepted for thermosets, thermoplastic elastomers and recycled materials.

For other applications, UL allows regrind up to a maximum of 25% by weight with the same grade of thermoplastic virgin at the same molder facility without further testing.   For regrind levels exceeding 25%, UL requires a special evaluation be done, which includes relevant performance verification tests such as Strength, Impact, Softening Temperature, Flammability, Ignition, Tracking and Long Term Aging tests. Information on evaluations of regrind percentages over 25% that have resulted in compliance is available on the UL iQ(TM) for Plastics database at ul.com/plastics.

In addition, UL's Follow-Up Service affirms the key controls in the molding system for the plastics.   This system ensures the proper usage of regrind, colorants, flame retardants, mold release lubricants and other additives which may affect the properties of plastics.

UL's Field Representatives have been working with manufacturers under UL's Follow-Up Service program for Molders to confirm the compliance to the regrind levels during their periodic visits.

What should molders do when using regrind materials?

• Use thermoplastic regrind materials in quantities only less than 25% by weight unless UL has verified that higher percentages can be used (see below)  
• If more than 25% regrind for thermoplastic materials is required, use only plastics grades available with higher regrind percentage as documented in the UL iQ(TM) for Plastics database at ul.com/plastics.
• DO NOT use any thermoset, elastomeric, or thermoplastic recycled regrind material
• Grind the material immediately upon processing or put the waste into a sealed container to avoid contamination via dust due to the static charges developed over the surface of the material.
• DO NOT use purged regrind materials.

What should end product manufacturers do when using parts made from regrind materials?

To comply with UL Follow-up Service requirements, if the molding takes place at the end product location, the manufacturing records should be made available to the UL Representative(s) upon their request to verify the percentage of regrind utilized.

If the molded parts came from an outside source not covered by UL's Follow-Up Service Program, a Certificate of Compliance (C of C) should be made available. UL Follow-up Service Filed Representatives will verify the regrind percentage information on the C of C. The information should declare that the reprocessed material content of the received thermoplastic molded parts does not exceed 25% by weight. UL's Representative(s) will also accept a procurement requirement on Purchase Orders to molders. A conformance statement should appear on the documentation accompanying each shipment. This documentation must be traceable to the individual lot(s) of the delivered molded parts. All verification information should be formatted according to accepted business practices and comprehensible to the UL Representative(s).

For manufacturers of ITE, the Web site at ul.com/ite/regrind.htm provides an overview of regrind thermoplastic issues related to the ITE Industry.