The processing of hemp is becoming increasingly important in Europe. Partly to compensate for climate-related losses in European flax production.

What are hemp shives?

Hemp shives are the woody, inner parts of the hemp stalk that remain after the bast fibers have been removed. They are light, absorbent, and naturally mold-resistant.

As a by-product, shives offer exciting applications, especially in hemp lime (hemp-lime building material).

The light hemp plant with its open cells:

In the long term, the lime reacts with CO₂ from the air (carbonation) and coats the shives with a mineral layer. This reliably protects them from mold, insects, and rot – even in humid climates.

The wide variation in the size of the shives poses particular challenges:

Coarse material is initially scanned at a high resolution (800 ppi) in order to capture even fine particles.

The resolution is then gradually reduced in order to hide details such as fine fibers and measure only the “stem” of the shive.

In the accepted-rejected (Figure 3), you can see that some shives are measured at approx. 300 ppi, while coarser material is only detected at 100 ppi. The FibreShape from IST – Innovative Scan Technologies GmbH adjusts these steps fully automatically.

When taking samples, care must be taken to ensure that the entire material mixture is recorded representatively. Since coarse shives often settle at the top and fine particles or dust at the bottom of the bag, stratification can occur, which distorts the result.

Weighting method

Due to the variation in length and width, a surface weighting (q₂) is used.

This ensures that each part is represented in the correct proportion to the total volume of the sample.

For synthetic fibers or very uniform natural fibers, on the other hand, a length weighting (q₁) is often useful.

Our expertise in measuring hemp shives is not only used for material characterization for customers. We also contribute our know-how to standardization committees such as ASTM in order to advance the industry as a whole through the standards developed there.

In chapter 4 the measurement system FibreShape is introduced. Get to know more about the analysis method and different fibre applications.

Find more information in the table of contents.

Curious to get to know more. The description of the FibreShape CROSS SC system gives you more insights.

The FibreShape system equipped with three different optical systems can fulfill this task. The scanners and microscope needed are described here. You find more information in the specifications of the FibreShape FH system.

In many sectors (chemical, pharmaceutical, textile or in the wood industry) quality standards need to be achieved. The results of our measurement systems are displayed according to ISO 9276 and DIN 66161.

FibreShape, PowderShape and DiaShape are measurement systems to analyse size, shape and other parameters of fibers, particles or diamonds.

What is to consider to make sure that results are representative and reproducible? For each measurement the following steps must be taken into account:

• Step 1: Take representative samples
• Step 2: Disperse the material sufficiently
• Step 3: Use the appropriate imaging settings. The images are generated by scanners or microscopes.
• Step 4: Use the appropriate measurement conditions (quantitative image analysis)
• Step 5: Choose evaluation according to the research question (for example distribution density)

The geometrical characterization of rCF fibers, roving snippets, yarns and nonwovens is done with the measurement system FibreShape. The geometrical characterization of fibers provides the information to predict the quality, strength and uniformity of a fiber reinforced composite.

Examples

1. Length measurement of single fibers (provided by the Faserinstitut Bremen)

The histogram of the distribution density shows a distribution from 0,025 mm to 1mm. In total 1972 objects were analyzed. The average fiber length is 0,2 mm. The control of the area ratio, in this sample 9,62%, ensures that the fiber density is not too high.

2. Roving snippets of rCF (provided by STFI)

The sample of the analyzed roving snippets counts 118 objects. The roving snippets have a length of 4 mm to approximately 13 mm.

3. RCF-nylon-hybridyarns (provided by DITF)

The average width (mean value) of the wrappers measured with the parameter “minor axis of Legendre ellipse” is 0,56 mm (+/- 0,15 mm). The distribution density of the angles of the wrappers vary from approximately -25 to 55. The value 0 represents the x-axis. The distances between the wrappers vary from 0 mm to 3 mm. $

FibreShape – quality measurement

FibreShape provides the user with the safety of a standardized quality measurement (1). Knowing the properties of a raw material is the first step to create a high-quality product. FibreShape can be used for entrance control of the length and width of rCF but also to control the fiber length as part of the processing chain (2).

FibreShape relies on scanners. Read more about scanners here.

rCF length of staple fibers

The fiber length is a key characteristic important in the production of roving snippets, yarns and nonwovens. When working with short-fiber composites it has to be considered that for the transfer of stress there is relatively little interface per fiber. The so-called “critical length” of carbon fibers is approximately 5 mm. This length is necessary to ensure that the fiber can be stressed to its full strength (3).

Depending on the compound, different rCF lengths are realizable. (Hybrid-)yarns from rCF or thermoplastic foils work with a rCF length of about 15 mm to 80 mm. In the case of injection molding, short fibers below 6 mm are used. RCF that result from pyrolysis can either be used for injection molding if of short length or for nonwovens in case of bigger length.

Literature:

1. ISO 9276 – 6, Representation of results of particle size analysis – Part 6: The descriptive and quantitative representation of particle shape and morphology, 2008.
2. Fischer, H.; Schmid, H.G., Quality control for recycled carbon fibres; Kunststoffe International 103,11, p. 68 – 71. ISSN 0945-0084. (2013).
3. Piggott, M.R., Interface Properties and their Influence on Fiber-Reinforced Polymers. Chapter 9 in Vigo, T.L. & Kinzig, B.J. (eds), Composite Applications – The Role of Matrix, Fiber, and Interface. Weinheim: Wiley-VCH, p. 221 – 265, (1992). ISBN 3-527-28094-4.

The FibreShape Automatic was used to assess fibre length and thickness distributions in recycled MDF: https://youtu.be/TXR8vDkFAQQ

More information about wood particle size measurement can be found in the following publications:

Burkhard Plinke, Jan Thore Benthien, Andreas Krause, Kim Christian Krause, Arne Schirp, Laura Teuber (2017): Optische Größenvermessung von Holzpartikeln für die WPC-Herstellung. Vergleich dreier Messverfahren.

Abstract: Optical size measurement of wood particles for WPC – Comparison of three methods

The production process and properties of WPC (wood-polymer composites) are strongly affected by the shape and size of the wood particles (e.g. length, diameter, the ratio of length to diameter) as well as frequency distributions of these parameters and the percentage by weight of specific mass fractions. For the time being, characterization of compounds and of particles used for the preparation of WPC is mostly insufficient and inconsistent. Suitable methods are the sieve analysis or optical measurements. Several optical measurement methods were evaluated using materials which are typical for WPC. The measurement systems FibreShape, QICPIC and additionally FibreCube were tested with wood flours with grain size varying from “very fine” to “comparatively coarse” and evaluated with respect to informational value, test duration, and applicability. All systems tested provided very similar results regarding the type of distribution and its characteristic percentiles for the particle length and a shape factor (length / width ratio) when optical resolution and weighting method were similar. However, mechanical as well as optical methods show constraints with respect to particle dispersion, measurement range and resolution. These constraints have to be considered. Optical size measurements are suitable for the characterization of particle size distributions and may be readily established in the process monitoring for quality of raw materials as well as in trade and standardization and can be included in the standard prEN15534-3.

Plinke, B., Schirp, A. und Weidenmüller, I. (2011): Methoden der Holzpartikelgrößenmessung – Von der technologischen Fragestellung zur aussagefähigen Statistik. In: 9. Holzwerk­stoff­kolloquium, S. 177-187. Institut für Holztechnologie, Dresden, 1.-2.12.2011.

Abstract: Methods of wood particle size measurement – From the technological question to informative statistics

The size distribution of wood fibres or particles as raw materials for fibreboards or particleboards is an important parameter for the production control. Size characteristics are currently measured using simple means such as sieve analysis which provides only weight fractions. However, contour details (e.g. length, width, aspect ratio of each particle) can be evaluated only by optical measurements. Today, automatic image processing methods for laboratory and on-line measurements are available which can be adapted to the material type. Shape measurements for manually separated particles together with weighted size distributions provide detailed information: E.g. the different lengths of particles from various wood species which originate from the same sieve fraction; the amount of shives and fibre bundles in MDF fibre material corresponding to the gap size in the refiner; different components (e.g. particles and fibres) in particle mixtures. The FibreShape system operated at WKI is a helpful tool for these investigations. In addition, using specific image processing procedures which were developed at WKI, it is also possible to recover and measure particle shapes in mat and board surfaces for a continuous monitoring of particle geometry in the production.

Further information concerning the measurement of wood particle size with FibreShape can be found at the Fraunhofer Institute of wood research.

Grain size or Min Feret diameter this are measurement values that can be identified for every single object (see image above). The grain size distribution is another value that will help you to better characterise your sample. To get to know more about the measurement conditions, settings and the interactive reporting system of DiaShape read:

DiaShape allows you to correctly measure size (~ carat), shape (~cut), color and transparency (~clarity) of industrial and natural diamonds, ABN (amorphous boron nitride) and CBN (cubic boron nitride) grits. You find more information and examples here.