PROJECT RESULTS

Use of Electron Beam Processing to Improve the Mechanical Properties of Flax Plastic Composites

Applicant: 

Terry Stepanik
Acsion Industries Inc.
Pinawa, Manitoba  R0E 1L0  Canada

Table of Contents:

• Background and Objective
• Procedure and Project Activities
• Results and Discussion
• Conclusion

ARDI Project:

#98-198

Project Status:

Completed July, 2000

Background and Objective:

Wood fibre-polypropylene composites (WFPC) have been used to manufacture interior parts for the automobile industry for over twenty years. The wood fibre imparts increased strength and thermal properties to the composite. It has long been recognized, however, that these properties are not optimized because the wood fibres and the polypropylene polymer do not bind to each other to any significant extent. Acsion personnel had previously shown that the mechanical properties of wood-polypropylene composites could be improved by adding chemicals to cross-link the wood to the polypropylene. Agrifibres such as flax and hemp have recently been investigated as substitutes for wood fibres in composites intended for automobile parts. As these materials are similar to wood fibres in composition, they would therefore be expected to impart similar properties to the composite, including the limitations discussed above with respect to binding.

The intent of this project was to determine if the experimental protocol used to improve the mechanical properties of wood fibre-polypropylene composites could also produce the same improvements in flax-polypropylene composites.

Procedure and Project Activities:

The experimental protocol consisted of mixing Durafill (flax shives) or Durafibre (flax fibre) with polypropylene, maleated polypropylene and a proprietary cocktail containing chemicals used to cross-link wood to polypropylene. This mixture was electron treated, melt-mixed in a twin-screw extruder, pelletized, then injection molded to form the test specimens used for tensile and flexural testing.

Sample sets containing polypropylene and flax fibre or flax shives were prepared using the proportions (by weight) shown below:

• 35% flax fibre: 65% polypropylene
• 50% flax fibre: 50% polypropylene
• 35% flax shives: 65% polypropylene
• 50% flax shives: 50% polypropylene

These sample sets also contained maleated polypropylene and a cocktail containing cross-linking chemicals.

Four batches of each sample set were prepared. Each of these sixteen batches were electron treated to one of four doses ranging between 5 and 15 kGy (the dose range shown to be most effective in promoting cross-linking between wood-polypropylene composites). An additional batch of each of the four sample sets was prepared containing flax, polypropylene and maleated polypropylene, but no cross-linking chemicals. These four batches were not electron treated and served as the untreated (0.0 kGy) controls. The resulting batches were extruded, pelletized and injection molded to produce the samples for mechanical testing.

Injection molding was performed at Plas-Tech Industries in Winnipeg. Mechanical testing was performed at Whiteshell Mechanical Testing, Lac du Bonnet, MB and at Acsion.

Results and Discussion:

Each injection produced four different specimens specifically designed for mechanical testing (Figure 1). During injection molding, carry-over or contamination between samples is inevitable when changing from one sample to the next. In addition, slight adjustments in the injection parameters may be necessary when changing from one sample to another to produce satisfactory specimens. Thus, multiple injections were made (up to 35 injections were made for each sample in this project) and the specimens used in the mechanical tests were those produced in the later injections as these were considered the best representatives for the sample.

Figure 1. Test specimens made with Durafill/polypropylene (50/50)

The tensile and flexural properties of the twenty-one different samples were determined. Five specimens were tested for each sample in each test. The tensile strength and tensile modulus data indicated that addition of the proprietary additive followed by electron treatment produced no significant effect on the tensile properties compared to the untreated control (0.0 kGy sample). Similarly, the flexural results showed no significant difference between control samples and samples containing the proprietary additive.

Conclusion:

The experimental protocol used in this project was developed at Atomic Energy Canada, Ltd. (AECL) and was successful in improving the mechanical properties of composites made from polypropylene and wood. Because the constituents in flax are similar to wood, similar improvements in mechanical properties might be expected for flax-polypropylene composites when this experimental protocol was used. However, the results indicated that this was not the case. It is possible that one or more of the parameters optimized for producing wood-polypropylene composites may not be optimal for flax-polypropylene production. Further development work would be required in order to obtain any improvement in the mechanical properties of flax plastic composites using this protocol.

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