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Wheat Straw in Construction Materials

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					Wheat Straw in Construction Materials
Presentation to
                                              nd
Industrial Uses of Wheat, Actin, Cambridge, 22 March 2000

Chris Phanopoulos, Christel Van Den Bosch, Ann Engelen and Lieve Vanoverloop
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Abstract of paper

A brief introduction of isocyanate chemistry will be given. Starting from the understanding of the
mechanism of adhesion of isocyanates to wood and an assessment of wheatstraw-board
performance, efforts will be made to investigate the performance behaviours and mechanism of
adhesion of isocyanate bonded wheat straw. Isocyanates bond to wood by a diffusion interphase
in which isocyanate has diffused into the wood structure at the molecular level and reacted with
water and some of the wood components, including lignin.

Isocyanates however, do not wet or penetrate well into wheat straw. Whilst adhesion with some
elements of the wheat may be strong, weak boundary layers as well as poor substrate integrity
seem to prevent good panel performance. Further, since the isocyanate is not penetrating into
the substrate it cannot consolidate the mechanical and biological integrity.

Some of the consequences of this adhesion mechanism plus some of the poor innate
characteristics of the wheat can thus explain the performance of bonded panels. These problems
are mechanical but also environmental such as poor thickness swell, poor resistance to microbial
attack.

A few possible methods to overcome these problems are suggested and provisional results
presented. These include mechanical changes to the processing such as tailored furnish refining,
as for example the removal of the outer protective layer of the wheat and the removal of high
silicon content leafy sheaths. Another example is in the use of coupling agents to improve
wetting or the use of chemical treatments to aid long term properties.
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Polymeric MDI, as supplied by Huntsman Polyurethanes, is a multifunctional relatively low
molecular weight mixture of aromatic isocyanates. It is a very reactive material, reacting with
hydroxylic materials, with water, with acids and even with itself and with its reaction products.
Due to the reactivity, most isocyanate processes are fast, and lead to a high molecular weight,
highly cross-linked plastics, see schematic 1.

Schematic 1. Typical forms of aromatic isocyanates and some of the common reactions


  OC N                CH 2              NC O               4,4’ M D I


                      CH 2              NC O
                                                           2,4’ M D I

                   N CO

  OC N                CH 2
                                                           2,2’ M D I
                                                           Di-isocyanates
                   N CO      N CO

  OC N                CH 2              NC O               Tri-isocyanate
                                                           and higher oligom ers
                                     CH 2
                                                           tetra, penta, hexa… ..
                                                 N CO      Polym eric M D I
Schematic 1 continued



 R                NCO + HO-R’             R                      NHCOOR’


R’ = aliphatic, aromatic, H                                  Carbamate
    (typically polyether, polyester polyol)                  “urethane”
    if R = H : carbamic acid


  R                NHCOOH                            R                 NH2 + CO2



  R                NCO + H2N-R’’                     R                 NHCONH-R’’


R’’ = aliphatic, aromatic, H                                  Substituted urea



 R             NHCOOR’ + OCN                  R’’        R             NCOOR’
                                                                       CO
      Carbamate                                          allophanate   NH          R’’




 R             NHCONH-R’’ + OCN                R’’       R             NCONH-R’’
                                                                       CO
        Substituted urea                                               NH          R’’
                                                             biuret


Extensive work has been conducted on elucidating the mechanism of adhesion of isocyanates to
wood and it has been found that isocyanates spread over the surface and penetrate rapidly
(figure 1) into the structure of wood. With sufficiently high concentration of resin, the penetration
causes a morphological change in the wood – but the exact nature of this change is not known.
This is exemplified in figure 2 in which the diffusion of isocyanate into wood and straw is shown,
after the initial uptake, wood shows a secondary increase in mass, attributed to a morphological
change. In this case, the uptake is subsequently followed by a loss in mass which is also
associated with the morphological change, in that the now continuous liquid isocyanate pathway
allows migration of wood extractives out of the structure, resulting in a mass loss. Once in the
wood structure, the isocyanate can react with various components (principally extractives and
lignins – some reactions are beneficial, but others are not) and with water. In this way, a diffusion
interface is established to hold adjacent pieces of wood together (figure 3). Since the isocyanate
bearing resin has penetrated extensively into the wood structure at the molecular level, the resin
not only causes the adhesion, but also modifies the nature of the substrate. The nature is
generally beneficially modified in that, properties such as water uptake and thickness swell, are
significantly reduced even at low resin loadings. It has been suggested that reducing the
thickness swell (and indeed retarding the onset of water induced swelling) aids in the retardation
of microbial infestation and growth.

Wheat straw is a cellulosic tube, composed of many layers. The quality of the furnish used to
manufacture a panel, is dependant on the refining strategy which causes variable damage and
damage type, varying ‘fines’ contents and so on. This may not seem significant, but fines have
been found, due to their high surface area, to require disproportionately high resin loadings in
general, and with wheat, the fines have been found to be composed of those components high in
silicone (table 1). Isocyanates do not wet these components well. And without good wetting,
good adhesion cannot be achieved.           The poorly wetted fine particles will then cause
imperfections in the glueline and reduce the quality of the derived boards.

Figure 1 Penetration of isocyanates into wood




Figure 2 Diffusion of pMDI into wheat straw and wood. The secondary uptake indicates a morphological change in the
structure of wood, which is not seen in the wheat.




  Mass increase/g



                                                         wood


                                                         wheatstraw



                                                                            -1/2
                                                                       Time/s
Figure 3 Modes of adhesion showing a diffusion interphase for the adhesion of isocyanates to wood and mechanical
interlocking for UF to wood.



   UF ADHESION TO                     MDI ADHESION TO
   WOOD: INTERFACE                   W OOD: INTERPHASE




Table 1 Characterisation of wheatstraw furnish, showing how ash content increases as particle size decreases and that
this is associated with poorer wetting by pMDI

   Part. Size     Content     % ash         contact angle
     mm             %                        m att (deg)
                     W heat Core

 1.00 - 2.00             2.02         8.3              52
 0.50 - 0.80            86.71         9.1              67
 0.25 - 0.50            10.21          11              84
 <0.25                   1.45        10.7 ?

                       W heat Face

 0.8 - 1.0                            7.9              58
 0.5 - 0.8                            9.8              68
 <0.25                               12.8              75

                      Single Culm

 stem (outer)                                          55
 stem (inner)                                          35
 sheath (outer)                                        50
 sheath (inner)                                        45
Therefore, it is recommended that, at least for panel production with isocyanate resins, the fines
be removed after refining. Alternatively, it is believed that removal of the sheaths and nodes of
the wheat prior to refining will reduce the fines formation and will ultimately improve bonding.

Despite this problem, isocyanates can stick wheat straw into useful panels. Typical properties of
laboratory made panels are given in the table 2 below. The mechanical performance is not
particularly good, but for a number of applications is adequate. It should be said that industrially
produced panels do generally give superior performances, with bond strengths typically above
0.6 MPa. Even so, the possible applications are all internal use – such as for furniture or
cabinets. But the thickness swell due to water ingress, could be improved and this disallows the
use of isocyanate bonded wheat strawboards in exterior or even semi-exterior applications.

Table 2 Typical performance of pMDI bonded wheatstraw panels

Sample number Resin Loading            Thickness        V20       24hr Thickness Swell
                    %                     mm           MPa                 %
      1             3.5                    11           0.45              20.8
      2             3.5                    11          0.494              20.43
      3             3.5                    11          0.621              20.68
      4             3.5                    11          0.599              19.9
      5             3.5                    11           0.43              20.38
   average          3.5                    11          0.519              20.44
     SD                                                0.087

        6                 3.5              18          0.423              18.1

    standard                                          0.5 - 0.6           <11

The nature of the interactions between isocyanates and wheat straw has been investigated in a
number of ways. Collectively the different assessments can provide insights into the true nature
of the interactions. Some of the investigations are presented in this paper.

Transmission FTIR spectra of ground wheat straw (heated and unheated), alone or mixed with
pMDI were recorded. In this way it was possible to see chemical changes occurring (i) within the
substrate due to heating, and (ii) between the resin and substrate. This showed that heating of
straw gave rise to no observable chemical changes, despite indications of significant structural
changes seen by SEM. Heating of straw showed evidence of increased surface debris and
changes in the shape of the cells. FTIR spectra of the substrate-resin showed changes
consistent with those seen for the heating of resin alone (with moisture). This indicates that the
curing process is only one of molecular weight build up and cross-linking of the resin. If there are
chemical interactions between the isocyanate and the wheat, then they are occurring at a very
low level. These changes are consistent with those seen with isocyanate and cellulose – no
                                                        1
reactions are seen. This finding is supported by Pizzi , he found that reaction of isocyanate to
cellulose only occurred at very high temperatures and only when there was no water available.

The only spectral change occurring which is not related to the water curing of isocyanates is seen
            -1
at 1000 cm (figure 4). The absorption bands around this wave number are characteristic of the
C-O stretches of cellulose. When first contacted with isocyanate, the intensity of the band
decreases and as cure progresses by exposing the mixture to heat, the intensity recovers to that
of the neat wheat spectral intensity. The cause of this is unknown; however, it is suspected that
optical effects (refractive index changes) are responsible which can distort spectra in this way.
Figure 4 Changes in the C-O stretch of wheatstraw cellulose in the presence of pMDI as the resin cures




                                                                        Increasing extent
                                                                             of cure
                   1000cm-1                     1000cm-1
                Wheat Straw         Wheat Straw + Isocyanate
Differential Scanning Calorimetry (DSC) which measures the heat changes at a range of
temperatures during reaction also showed that no reaction was occurring between the isocyanate
and the wheat substrate (figure 5). With open crucibles, water was preferentially lost due to
                                                 o
evaporation and only after heating beyond 200 C were there any signs of chemical changes.
However, it is difficult to deduce what these may be, but, usually at these temperatures, thermal
decomposition of organic materials is occurring.

Figure 5 DSC thermograms of pMDI-wheatstraw mixtures in open and closed crucibles
 Heat flow




             Open Crucible




                                100oC                        >230oC

             Closed Crucible


                                           130 - 150oC
                                                                   Temperature


In a close crucible on the other hand, a characteristic isocyanate-water reaction exotherm can be
seen. Again suggesting that polyurea formation is the only reaction occurring.

Kinetic studies of the wheat straw – isocyanate system support these findings of lack of reaction.
The kinetic data (measured in three different ways – by DSC, by a modified method developed by
      2
Chow and by FTIR) show that isocyanate-isocyanate reactions and isocyanate-water reactions
occur preferentially and more rapidly than possible isocyanate-wheat reactions.

Since there was very little evidence of any chemical interaction between the isocyanate and
wheat straw, physical interactions also needed to be checked. It was not possible to assess
penetration as done for wood using the optical microscopy method developed for wood, however,
the use of x-ray microscopy in the future may be considered. However, indirect evidence of
penetration can be gained from the study of diffusion of isocyanates into the straw (figure 2). For
this, single crushed strands of straw are immersed in the isocyanate and the mass periodically
recorded (care being taken to remove excess surface resin). The mass increase as a function of
                                                                          nd                 3
the square root of time, then gives the diffusion profile. Using Fick’s 2 law of diffusion , it is
possible to calculate the coefficient of diffusion. The first observation is that the uptake of
isocyanates by straw is significantly different from that seen with wood.

Wood shows, a secondary uptake (extent and time are dependant on temperature and species).
This uptake is due to a morphological change in the wood, which is induced by the presence of
the isocyanate. A certain concentration of isocyanate and a certain time which is dependent on
the relaxation processes induced by that concentration of isocyanate are needed for the
morphological change to occur. It is not known what this change is, however, it is likely to be due
to the non-cellulosic content of the wood. Further, it is not known whether this change is
beneficial or not for good adhesion and performance development. However, since the wheat
straw does not experience the change, the mechanism of adhesion must be different.

All the above, whilst not comprehensive, indicate quite strongly, that adhesion of isocyanates to
straw are physical in character, and probably it is a mechanical interlock. That is not to say that
the adhesion is weak, but the presence of weak substrate interlayer links, limits the overall
performance of the derived panels.

Some of the properties required for the introduction of wheat boards into exterior applications
were evaluated. These include bio-resistance and long term mechanical property retention.

Microbiological resistance was studied by subjecting panels to closed environments which were
                                                                                o
maintained at high relative humidity (>70% RH), modest temperature (30 C) and which were
seeded with a ‘cocktail’ of common fungal spores. Polymeric MDI bonded wood panels fared
better than PF bonded panels, but under these aggressive conditions, all became infected with
fungal growth. The pMDI-bonded panels did show a delay in infestation compared to the PF, but
once infected, growth rates in both cases were equivalent. Straw panels were found to become
heavily infected very rapidly, with extensive fungal growth covering the whole panel within days.
However, it was found that the colonisation was not with one of the seeded fungal species. It was
suspected that the fungus was already present in the furnish and the storage conditions allowed
                                                o
rapid growth. The panels were made at 200 C albeit for a short period of time and it is assumed
that all biological material is destroyed at these temperatures, except for fungal spores which may
be present. These then obviously germinate in the storage conditions and rapidly grow.

Several methods to improve performance were assessed. These included the post treatment of
panels with a coating of pMDI (figure 6), the use of fungicides (figure 7) and pre-acetylation of the
straw furnish (figure 8). Acetylation, as shown in figure 8 is an effective, but expensive method to
improve microbial resistance. Coating with a layer of pMDI did not improve resistance to any
measurable degree, nor indeed did the employment of unreacted isocyanate groups show any
advantage.

Figure 6 Impact of pMDI coating film on microbial resistance of strawboard
   pMDI coated strawboard, unexposed               pMDI coated strawboard, exposed

Several fungicides were found to be beneficial in terms of preventing fungal infestation, as
indicated in figure 7. However, significant care is needed in the selection of a biocide as they
may chemically interact with the isocyanate resulting in the deactivation of one or both of the
components.




Figure 7 Impact of fungicide on bio-resistance of strawboards




Untreated pMDI bonded straw              2% o-phenylphenol treated pMDI bonded straw

Figure 8 Impact of acetylation of straw on bio-resistance of strawboard
                          Acetylated straw                  Natural straw

Retention of mechanical properties was studied by monitoring the internal bond strength of wheat
straw panels stored in a variety of conditions. In dry environments (<40%RH) there was no
observable decline in performance as would be expected. However, in constant wet conditions
(>90%RH), properties very rapidly declined. The drop in mechanical performance was attributed
to loss of substrate integrity due to fungal infestation and was not seen as being due to a loss of
adhesion. Finally, panels stored outside (North Europe, south facing, uncovered table) showed
immediate, relatively rapid decline in performance. The drop in performance was especially
noticeable after prolonged exposure to rain with only minimal (incomplete) recovery after long dry
periods.

Finally, some options to help improve these poor performance characteristics are being looked at.
These include, ways of improving substrate wetting, modified wheat straw refining, and chemical
treatments of the substrate.

The impact of acetylation was covered above. Removal of the nodes and sheaths was found to
enhance adhesion and mechanical performance and finally, the use of coupling agents. Several
families of coupling agents are commercially available, shown below (figure 9) are the impact on
performance of two types from the same family. As can be seen, the internal bond strength
improves to above requirements. Thickness swell also gets better marginally despite increases in
the extent of water uptake. The modulus of elasticity however is detrimentally impacted.

Figure 9 Use of coupling agents in pMDI bonded strawboard performance (a) water uptake, (b) thickness swell, (c)
internal bond strength and (d) modulus of bending


                     60                                                                           22
                                                                            Thickness Swell (%)




                     59
                                                                                                  21
  Water Uptake (%)




                     58
                                                                                                  20
                     57
                     56                                                                           19
                     55
                                                                                                  18
                     54
                                                                                                  17
                     53
                     52                                                                           16
                                pMDI             75.1       75.2                                             pMDI       75.1       75.2
                                             Resin System                                                           Resin System


                          (a)                                                                          (b)
                                 0.7                                                                    3300
  Internal Bond Strength (MPa)




                                                                          Modulus of Elasticity (MPa)
                                                                                                        3250
                                 0.6
                                                                                                        3200
                                 0.5                                                                    3150
                                 0.4                                                                    3100
                                                                                                        3050
                                 0.3                                                                    3000
                                 0.2                                                                    2950
                                                                                                        2900
                                 0.1                                                                    2850
                                  0                                                                     2800
                                             pMDI       75.1       75.2                                          pMDI       75.1       75.2
                                                    Resin System                                                        Resin System


                                       (c)                                                                 (d)

Conclusions

The production of wheat straw panels bonded with pMDI is of acceptable mechanical quality for
interior applications. However, they suffer from inadequate mechanical performance and
inadequate durability and resistance to microbial attack to allow their employment in exterior
locations. There are on the other hand indications that solutions to overcome these performance
deficits are possible and achievable. These include the use of coupling agents, biocides and
optimisation of chemo-mechanical refining. To achieve this multi-partner alliances would be
needed to best reach the targets.

References

1. A. Pizzi and N. A. Owens
         Holzforschung 49(3), 269-272, (1995)

2. S. -Z. Chow
         Wood Sci. 1, 215 (1969)

3. J. Crank
                                       nd
         The Mathematics of Diffusion 2 Edition Clarenden Press Oxford (1975)

				
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