For decades, makers of polyethylene wire and cable, shrink tubing, hot and cold water piping, sheet, and foam have used radiation crosslinking to enhance the thermal and mechanical properties of their products.
For decades, makers of polyethylene wire and cable, shrink tubing, hot and cold water piping, sheet, and foam have used radiation crosslinking to enhance the thermal and mechanical properties of their products. Some resin suppliers and a small number of large processors have also irradiated PE pellets before extrusion or molding as a way of custom tailoring their processing and/or performance characteristics. Most often, that irradiation is performed on a toll basis by contract service providers. Now there’s a new wrinkle: A contract radiation processor is selling its own brand of radiation-modified PE pellets.

This novel approach is being taken by Sterigenics International, which claims to be the world’s largest provider of contract radiation services, with 17 plants in North America and three in Europe and Asia. After several years of processing pellets for individual customers, Sterigenics came out with its Raprex brand of HDPE and LLDPE pellets in 2004.

According to Lew is A. Parks, v.p. of Sterigenics Advanced Applications, the company initially thought pro cessors would buy PE resins and send them for toll processing by Sterigenics for around 5¢ to 25¢/lb. But processors showed more enthusiasm for buying pretreated pellets that came with datasheets of specified properties.

Demand has taken off so successfully, says Raprex product manager George Forczek, that Sterigenics is expanding its capacity to handle larger orders. The company has dedicated a facility for irradiating pellets at Gaithersburg, Md. Available to begin processing in the first half of this year, it will accommodate truckload orders initially and railcar quantities later on. Throughput rates will be up to 20,000 lb/hr. Forczek envisions multi-million-pound annual sales of Raprex within the next few years. Beyond polyolefins, other radiation-modified thermoplastics are in development.

 

What radiation provides

Sterigenics uses gamma and electron-beam (EB) radiation to modify polyethylene. According to Parks, the “recipe” for Raprex products includes selecting the proper PE base resin (with special attention to additives), choosing the type and dosage of radiation, and compounding stabilizers and other additives after radiation. Radiation treatment provides three effects:

  • Increased chain branching and broader molecular-weight distribution.
  • Oxidation, which adds polar end groups to the base polymer.
  • A low degree of crosslinking, which does not harm recyclability.

These radiation effects produce three main benefits: First, processability is enhanced because viscosity is increased at lower shear rates. That means greater melt strength (handy for extrusion) without greater flow resistance (good for molding).

Second, polar end groups provide better adhesion to metal for pipe coating and to fillers like wood flour. 

Third, a variety of benefits derive from improved end-use properties such as mechanical strength, toughness, wear resistance, ESCR, HDT, flame resistance, and heat-sealability.

Sterigenics offers three grades for injection molding and extrusion:Third, a variety of benefits derive from improved end-use properties such as mechanical strength, toughness, wear resistance, ESCR, HDT, flame resistance, and heat-sealability.

  • Raprex 100 extrusion-grade HDPE is aimed particularly at water and pressure pipe, as well as profiles. It boasts higher stiffness, burst and hoop strengths, and ESCR.
  • Raprex 200 HDPE for injection molding has better strength, toughness, heat resistance, and flame resistance.
  • Raprex 300 LLDPE for blown film has higher tear and puncture strengths. It is aimed at mono- and multi-layer greenhouse film, geomembranes, and industrial liners or covers.

 

Other service providers

Probably the oldest and best-known U.S. name in radiation modification of plastics is E-Beam Services, formerly the radiation processing unit of Monsanto. The company has three facilities in New Jersey, Ohio, and Indiana. John Messick, national sales manager, says the firm processes millions of pounds per year of pellets on a toll basis for resin producers and a couple of processors that mold filter elements. E-Beam also treats finished products such as gaskets, pipe, tubing, or sheet. He says upgrading heat resistance is the most common goal sought from radiation modification.

A large supplier of contract sterilization, with at least a dozen radiation-processing facilities, is Steris Isomedix Services. At its EB facility in Libertyville, Ill., the firm modifies plastic pellets and parts (wire, cable, and tubing). EB sales application specialist Chad Rhodes says Steris provides toll treatment of HDPE, LDPE, and EVA pellets for resin companies and a few end users.

 

TABLE 1—RAPREX 100 IRRADIATED HDPE FOR PIPE
(12.8 mm Diam., 1.9 mm Wall)
Density, g/cc0.950
Melt Index, g/10 min
190 C, 21.6 kg

3.5
Tensile Strength, psi
  Yield
  Break

4350
3973
Elongation @ Break, %506
Flex. Strength, psi
  212 F
  -40 F

1454
1380
Burst Strength, psi
  73 F
  212 F

1412
660
Hoop Strength, psi
  73 F
  212 F

4046
1885
Impact Strength
  73 F
  40 F
  Aged @ 257 F

No Cracking
No Cracking
No Cracking
Stress Cracking, psi @ 140 F1431
Chemical Resistance
  Ethanol (200 hr)
  Battery Acid ( 70 hr)
  Oil (70 hr, 212 F), % Change

No Cracking
No Cracking
3.98

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE 2—RAPREX 200 R-200C1
IRRADIATED HDPE FOR INJECTION MOLDING
Property
Non-Irradiated
Irradiated
Density, g/cc0.9520.950
MFR, g/10 min
  190 C, 2.16 kg
  190 C, 10 kg

0.75

<0.10
1.10
Tensile Strength, psi
  Yield
  Break

4200
1880

5650
3040
Elongation, %
  Yield
  Break

8
106

9
20
Flex Modulus, psi2465
Notched Izod Impact,
kJ/m2
50/No Break54/No Break
HDT, F @ 66 psi165189

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE 3—RAPREX 300 R-300A1
IRRADIATED LLDPE FOR BLOWN FILM

(25-Micron Film)
PropertyNon-IrradiatedIrradiated
MFR, g/10 min
  190 C, 2.16 kg
  190 C, 10 kg

1.01
7.69

0.20
3.46
Tensile Strength, psi
  Yield MD
  Yield TD
  Break MD
  Break TD

1421
1189
5133
4046

2567
2045
7366
6670
Ult. Elongation, %
  MD
  TD

600
710

750
790
Tear Str., g/micron
  MD
  TD

7.0
7.7

12.2
13.4
Puncture Str., g481921
HDT, F @ 66 psi109109