Thursday, 22 March 2018

Cycling the Golden Gate Bridge in San Francisco

By Author

I was going to title this post cycling in America but that would have been misleading. I’m not even cycling in San Francisco for real, as much as my cycling heart and trembling thighs would like to scale the beautiful hills of this lumpy city. I’m on holiday you see and so I’m on a hire bike taking on one of the most picture perfect postcard cycle rides of a metropolis the world has to offer. No, I cannot complain.

Cycling over the Golden Gate Bridge to Sausalito

How far is it, was the question my girlfriend should have asked but didn’t. The ride is eight miles and I was surprised to get away with such a ride on our holiday. Even then, she loved it despite a mini meltdown in the wind towards the end of the ride over the bridge.
I loved it of course. You really do appreciate the art deco detail and stylings when cycling along this engineering marvel.

Riding the famous bridge

My stomach feels fine. You?
My stomach feels fine. You?
As famous for its colour and architecture as for its suicides, the Golden Gate Bridge is a marvel of engineering. More than that it is beautiful. Especially when the rolling fog clings to its underbelly or shrouds its peaks in mystery.
The ride out to the bridge is nice and flat but for one small climb at the very start of the bridge. Don’t worry. Stick the hire bike in the granny gear and you can mount any climb in San Francisco.
At the top of the climb, there’s a highly recommended diversion down to Fort Point. A roll down this hill will give you some of the best vantage points and best views of the Golden Gate Bridge. Oh, and some crazy surfers waiting on waves oh so very close to the rocky shoreline. Seriously dude, I mean WTF.
The ride over the bridge is a pleasurable 1.7 miles (or 2.7km if you swing that way). There’s a little pedestrian dodging and a few over-serious gents on road bikes to avoid but the sheer length of the bridge gives you plenty of time to admire its sharp lines and steel cables, not to mention the fog and views of Alcatraz and downtown San Francisco.

Is it worth cycling over the Golden Gate Bridge?

A bridge to another world. Or Sausalito.
A bridge to another world. Or Sausalito.
Are you crazy? Yes. Beg, buy, borrow or steal a bike if you must, just do it. Besides, there’s not a lot else to do in San Francisco. Sorry San Fran, you’re just not my cup of tea. Or joe for that matter.

Tips for cycling the Golden Gate Bridge

  • Wear layers. It may be warm in the city sun but once you’re up on the foggy, windy bridge you’ll be thankful for the extra layer or two.
  • Stay for lunch in Sausalito. This little bay side town is quaint and there’s some good eating with waterside views. I’d recommend Salito’s Crab House and Prime Rib.
  • Return via the ferry. Buy your ticket before you sit down to lunch to avoid the queues. The ferry ride back has great views of Alcatraz and the city.
  • Look out for seals. Cute, if that’s your thing.
  • Take a detour to Fort Point for the best views of the Golden Gate Bridge.
  • Get a cycle route map. Should you venture away from the bridge, this map handily shades the hills of the city according to the severity of the slope, meaning you can avoid or seek the famous hills as you please.
  • Go see the Bay Bridge. It gets lonely but is of equal stature to the golden child, see below.
  • Lock your bike in the city. San Francisco is home to more homeless people than any other city in the USA*. This you’ll know the minute you begin walking the streets. We saw a group of homeless folk in the Tenderloin admiring their haul from the day, lots of shiny bikes. This was no sportive we’d stumbled upon.
*America, land of the free, home of the brave but no place to be should your luck run short. The number of homeless people in this city should be a national embarrassment. I’m sorry USA, but I’m embarrassed for you as a human being. Still, at least we can comfort ourselves that the destitute will soon die and halve the number of people sleeping on the streets.

A word for the poor old Bay Bridge

Not the Golden Gate Bridge. Bay Bridge. Look at me, please look at me.
Not the Golden Gate Bridge. Bay Bridge. Look at me, please look at me.
Nobody rides the Bay Bridge, the ugly sister in a town revered for its golden princess. Formly known as the San Francisco–Oakland Bay Bridge, this grey bridge was built before its more illustrious rival, is longer, holds the record for being the world’s widest bridge, is beautifully lit at night by fancy LEDs and… Wait for it… And it takes you to Treasure Island. No kidding. And yet nobody loves the poor old Bay Bridge.

Renting a bicycle in San Francisco

Cycling the hills of San Francisco
Rent and hire. Just one of the many cycling Americanisms. Seat and saddle. Tire and tyre. Spandex and Lycra. Fenders and mudguards. Hex key and Allen key. Lance Armstrong hero and Lance Armstrong doper.
Sure, I could have gone to Fisherman’s Wharf and paid a small fortune to rent a bike but hey, I’m in San Francisco and I care about the world. And I’m tight. So I headed over to the non-for-profit cycle hire shop called The Bike Hut near Mission Bay.
A bit of a trek from my place, I’d recommend a bus, but this option is much cheaper and you’re helping young people into work. We were handed our bikes and told to pay upon our return. No deposit. We cycled off, a little worried for the trust fund being put aside for the young people hoping to find work. Maybe I’ve got a face you can trust.

Cycling in San Francisco

Some observations. A great number of cyclists heave up the city’s 40 plus hills, a great sign of progress for American cycling. The diversity of riders is also very pleasing. Gone is the male dominated Lycra set of London, replaced with all comers, male and female, old and young.
Such scenes do make me worry for the so called cycling revolution in my home city, where cycling is perhaps more of a commuting necessity for those brave enough to take it on than a lifestyle choice. You can’t have a cycling revolution with such a small segment of society.
Amazingly the fixed gear / single speed bicycle appears in great abundance on the inclines of San Francisco. I’m guessing the single speed predominates simply because I ride fixed on the flats of London and I’m not sure a fixie is the wisest of choices in San Francisco. If you find a gear easy enough for the climb then good luck descending and vice versa.
Whilst some of the bike choices appear to be hipster posturing, a lot of the bikes have a very practical feel to them with wide tires and racks on the front and back. High five San Francisco, cycling is most definitely alive in the city. It was a pleasure riding you. Ahem.
For further information log on information :

Fixed gear bikes – How, why, when, what, which

By Author

Fixed gear track racing smallFixed gear. You and the bike are one, brain gives way to cog, crank arms are mere extensions of your legs and vice-versa. You are a cycling cyborg, part human, part machine, unsure where the steel ends and the flesh begins. Only the smile on your face reveals the human. Riding a fixed gear bike is special. This much you’ll discover the first time you ride fixed and stop pedalling.
The best things in life are the simplest. Extravagance is an absurdity best left to those vainly searching for happiness at the bottom of their wallet. Silence. Starlight. Sunrise. The sea. The touch of a loved one. The smile of a stranger. A Bob Dylan lyric. A cup of coffee. Bubble wrap. A shed. Single speed bikes. One cog, one gear, many cadences.
Riding a fixed gear bike is cycling in its purest, simplest form. Reducing life to its essential elements reminds you what it is to be human. Cooking with fire. Being naked. Although not necessarily at the same time. The less we have, the less we stand to lose and the less we can be distracted. Riding fixed, dérailleurs cannot break and gears cannot slip.
“The greatest wealth is to live content with little.”
That’s not to say Luddites should rule the earth. I welcome change and progress yet it is easy to forget the origin of why we began to evolve in the first place. Rarely do we look upon fire, the wheel, and the bicycle with the awe of when they were first discovered or invented.
Progress isn’t always essential despite the claims of the marketing men and women who strive to make anything seem necessary. Electronic gears. Cycling specific socks. Energy gels with added Oompa Loompa testosterone. To ride fixed is to strip all of this back and ride a bicycle as it was intended. Pure simplicity in itself.

What’s the difference between a single speed, a fixed gear and a track bike?

The track stand haunch a.k.a pointless perching
The track stand. Supposedly cool, definitely inefficient
First, let’s avoid any confusion. Not all single speed bikes are fixed gear bikes. A single speed bike can either have a fixed gear or a freewheel i.e. you can roll without pedalling. A fixed gear bike is a single speed on which you must always pedal. Why should you care? You shouldn’t, unless you’re about to ride 100 miles and have a choice between the two!
What about a track bike, what’s that all about then? A track bike is a fixed gear bike. Simple. Or is it? Track bikes must meet stringent regulations to actually be ridden on a track. This includes a high bottom bracket clearance and a short crank arm length enabling you to negotiate the banked curves of the track without striking the pedal.
More importantly, a track bike cannot have brakes if used for its intended purpose. Brakes on a track are considered dangerous when everybody else is riding brakeless. If you want brakes on a track bike you better come equipped with a drill. Finally the geometry of the frame is engineered for the pure speed of the track so a track bike will ride a little harsher on the road and a shorter wheelbase means potential toe overlap on the front wheel.

Benefits of riding fixed gear

Clean lines. Repaint required after every wet ride.
Clean lines. Repaint required after every wet ride.
  • Low maintenance. This was the primary motivation for my fixed purchase. Nobody enjoys cleaning chains with a toothbrush or digging out grime from a rear cog. When the fear of getting your bike dirty prevents you from riding you know you’ve got issues. Muck aside, single speed bikes have fewer parts ergo less to break or adjust. Commuting you no longer need worry about potholes damaging the wheels of your best bike either.
  • It’s fun. This is why I would recommend a fixed gear bike. Prior to becoming a fixed convert I read so much about the supposed fun of riding a fixed gear bike and was very much what you would call my usual cynical self. How on earth could being chained to the revolutions of my cogs be so much fun? The only answer to this question is to ride fixed. There is no other explanation. Forget all of the mystical stuff, the being at one stuff, the pure cycling stuff. Just go for a ride and note the smile on your face and then you try to explain the fun. And no, fun should never need explaining.
  • Simplicity. Not having to think about changing gear frees the mind to enjoy the ride. One less distraction. It’s you, the bike and the open (hopefully flat) road. No more crunching of gears as you accelerate away from the traffic lights. Free of gears, you’ve also shed some serious weight from your rear wheel. Light and sprightly you are.
  • Fitness. There are many spurious claims as to the fitness benefits of riding a fixed gear bike (see fixed gear myths below) yet one thing is undeniable: you must pedal at all times. No coasting means extra effort, assuming that is you have chosen the correct gearing.
  • Aesthetics. Only a philistine can shrug with indifference at the beautiful clean lines of a fixed gear bike. This is perhaps why the so-called hipster has jumped on a single-speed and turned the bike into something of a fashion statement. Shame the irony of the multi-coloured deep rims and tyres is seemingly lost on their aesthetic sensibilities.

Will riding a fixed gear bike improve my pedalling technique and strength?

Massive cyclist thighs
Before and after you started riding fixed
Yes and no. It will not teach you some mystical pedalling technique because let’s face it, pedalling is pretty easy. You pedal in a circle, job done. Will it teach you to pull up on the pedals? No more than cleats and being fixed to your pedals would. Some say riding fixed actually makes you a lazier rider when pedalling as the motion of the chainring does the work for you.
I’ll get stronger though won’t I? Maybe, depending on which gear you choose, which is the same as riding a road bike in a gear that’s a little harder than you might normally manage. So no, a fixed gear bike per se will not improve your strength or power outputs.
So what will riding fixed do for my cadence? Well it will teach you to pedal all of the time. No coasting. Freewheeling quickly becomes a thing of the past. Since riding fixed for my commute, on long weekend rides on my road bike I find myself consciously forcing myself to stop pedalling when going downhill in a bid to save energy. Never coasting is a benefit and a curse depending on the type of ride you are on. Not convinced? Ask a seasoned road bike century rider to complete a hilly 100 miles on a fixed gear and take one look at their post-ride face!

Tips for riding a fixed gear bike for the first time

Fixie fan Jesus learns a new trick to add excitement to his tired water becomes wine routine
Fixie fan Jesus learns a new trick to add excitement to his tired water becomes wine routine
If you’re used to riding a bike with a freewheel i.e. the bike rolls without you pedalling, then you’re in for a surprise the first time you ride a fixed gear bike. Without being overly dramatic, you pretty much have to learn how to ride a bike again. OK, so that was a little melodramatic yet it is not far from the truth. Here’s some tips on how to ride a fixed gear bike:
  • Pedal. At all times. You are the cycling equivalent of a shark, a prehistoric beast that must always keep moving to stay alive. Stop pedalling at your peril.
  • Cornering. Many a new fixed rider fears tight corners and pedal strike, that moment when pedalling around a corner your pedal hits the curb or floor at the low point of the pedal stroke. In over 1,000 fixed gear rides I’m yet to encounter pedal strike. Have I found a mystical Roman road with no corners? Not exactly. My fixed bike is a track bike (hey, I liked the colour!) and thus the bottom bracket is a little higher from the floor. Added to that are my shorter than normal crank arms at 165 mm instead of the 172.5 or 175 found on most road bikes.
  • Fit a front brake. I cannot stress the importance of this enough. Hence I go on about it quite a bit further down. You are not a better rider if you ride brakeless. You’re just slower, more tired and more of a danger to yourself and others around you.
  • Don’t worry about a rear brake. That’s called your legs.
  • Don’t learn how to skid. You’ve a front brake, you’ll never need to skid. OK, maybe for fun. Once.
  • Learn to track stand. The art of track standing will improve your balance and give you more control over the bike at very slow speeds.
  • Learn when not to track stand. Which is most of the time. You may think you’re cool track standing at the traffic lights for three minutes but you look ridiculous. Besides, it’s a complete waste of energy. But you’ll get away from the lights quicker right? Maybe. Or maybe you have an overgeared bike and so gaining momentum is oh so slow and you’ve been passed by everyone who pushed off the floor with their feet and clipped in quickly with their double-sided cleats. Track standing at busy ASLs in close proximity to other cyclists is also dangerous as you edge to and fro into the line of other riders.
  • Strap yourself in. When your pedals move regardless of whether your legs are moving, you don’t want to find your feet slipping or being thrown from the pedals. Ever tried to reconnect your feet to a pair of pedals spinning at 100 rpm? Good luck!
  • Learn how to start from a stop position. Assuming you’ve realised track stands are well, for the track, you’ll need to either i) come to a stop with the pedals in a position that allows you to accelerate (so leading foot positioned between 1-3 pm on a clock face) or ii) use your front brake to lift the rear wheel the 1 mm off the ground required to quickly spin your leading foot into position – a trick that you will soon master in the blink of an eye.
  • Learn how to spin downhill. I’m not going to lie. I look and feel ridiculous spinning away at 140 rpm down a steep hill. Control over the bike is also more difficult. Embrace the spin or alternatively learn how to control the bike and roll gently down a hill. Again, here’s where brakes help.
  • Increase your vigilance. Even though I ride fixed with brakes, I have found I have a better awareness of everything happening around me. As mentioned above, riding fixed is like learning how to ride all over again and so you begin to appreciate the flow of the roads much more, slowing down by leg power rather than caliper power. There’s something about riding fixed that makes you never want to stop and so you’ll pace yourself towards traffic lights so that you’ll never lose your momentum (without becoming a red light jumper).
  • Choose gears and routes carefully. You don’t want to be undergeared and spinning away any more than you want to be overgeared and mashing your way up hills. More on gears further down the blog.
  • Get the right chain tension and a straight chainline. This simply means getting the chain tension nice and tight. Not too tight otherwise you’ll find turning the cogs a little stiff. Too loose and you risk serious injury should your chain jump off. The chain should be as tight as possible whilst still allowing free movement of the drive chain. A straight chainline will ensure 100 percent connection with the chain and cogs, hence they’ll be less likely to slip off.
  • Bunny hop whilst pedalling. A difficult but must-have trick for fixed gear bike riding. Forget track stands and skidding, the bunny hop is the trick you need to beat those pesky potholes. Start small and move slowly before working your way up.
  • Beware of flares and laces. You don’t want anything getting caught in a fixed gear chain. Oh no.
  • Look cool. You’re riding a single speed bike, you must be a hipster, right?
  • Don’t ride like a moron. Excuse my language, but there does seem to be a strong correlation between the number of singlespeed (not necessarily fixed gear) riders and those with a blatant disregard for every other road user. Sure, cyclists riding all sorts of bikes can be idiots yet my experience here in London tends to lean heavily towards those riding a single speed.

Can I ride fixed gear without brakes?

Who needs brakes when you've got walls?
Who needs brakes when you’ve got walls?
Sure. You can also jump out of aeroplanes without a parachute or ride a motorbike naked. I just wouldn’t recommend it. Yes, your legs can act as brakes. So too your head when you hit the back of a bus that stops suddenly. But, but, my legs are really strong. Sure, rely on your legs to stop you, just make sure you quadruple your breaking distances and don’t travel over 10 miles per hour.
Ah yes, but I know how to perform skids on my fixed gear. That may be, but if you’re skidding then you’re not riding a big enough gear to get a decent speed (at least not for me!) and you had better have a good supplier of tyres and more money than sense. Finally, remember that riding a fixed gear bike without brakes is illegal here in the UK unless you’re on a track.
A front brake is a must for me. My fixed is my commuting bike here in London, a city where a brake is as essential as sharp elbows and the ability to avoid eye contact during rush hour on the Tube. Brakes save me energy as I no longer need to pedal backwards to slow myself down and I can also cycle much faster safe in the knowledge that I can stop as quickly as any other bike on the road.
Does a front brake really upset the true beauty of your bike and the ride? No, you only use it when you need to and from an aesthetic point of view, most people who chat to me about riding fixed at traffic lights ask if my bike has brakes, so discreet is my set-up.

Are fixed gear bikes dangerous?

Baggy pants and a fixed gear bike. Good luck.
Baggy pants and a fixed gear bike. Good luck.
Bikes of any form are not dangerous, riders are. Follow the tips above, use your front brake and you’ll be fine. Are fixed bikes safer than road bikes? No safer, no more dangerous. You could argue the chain is less likely to fall off, but when it does you will certainly know about it.
You’ll need to relearn how to ride a bike without coasting otherwise it’s buckaroo time and a chance to test your rodeo skills, which you will almost certainly experience when first climbing on a fixed gear bike. After that, the greatest danger comes in behaving like an idiot, but it was ever thus.

Is riding fixed bad for my knees?

Only if you ride the wrong gear, pretty much like selecting the wrong gears on a road bike. You don’t want to be mashing away on a huge gear and neither do you want to be spinning a tiny gear. Much depends on where you ride. If you’re in a hilly part of the world then a fixed gear bike may not be for you (although judging by the number of fixed gear bikes I saw in San Francisco , much also depends on your fitness).

Should I buy a fixed gear bike?

This very much depends on the riding you want to do. Flat commute? Go for it. Hilly, long distance Audax rides? Probably not unless you like suffering. Horses for courses, as any amateur gambler knows. Want to look cool? Sure, go for it but you can achieve the same riding a single speed with a free wheel (see obligatory fixie hipster slurs below). Either way, some will label you for riding a bike with one gear but then such things are not a worry to anybody wearing a moustache or Lycra, or anybody who rides what the hell they want to ride!
I spent months debating whether to go fixed or freewheel, mostly because riding fixed seemed very daunting, plus I knew I would have to go clipless too, something I swore never to do commuting in London. My worries quickly dissipated once I began riding. Sure, there’s an initial learning curve but once overcome you will wonder how you cycled for so long without enjoying the purest form of cycling pleasure there is; riding fixed.

I thought fixies were for hipsters?

Travelling through a red light near you soon
Travelling through a red light near you soon
I cannot write a blog on single speed bikes without mentioning the H word. Sure, the bearded men and moustachioed women of downtown Dalston here in Londinium may prefer to ride a bike with one gear, but observe closely and you’ll notice that 47 times out of 48, the bike has a freewheel and is a single speed rather than a fixed gear bike. Boat shoes and fixed wheel bikes don’t tend to get along too well.
Anyhow, who cares? Cycling is meant to be fun. Ride what makes you smile. You’re a cyclist, since when has style or fashion come into your decision-making process? You live for the days when snot runs down your face, the interval sessions that turn your head beetroot, or when the rain soaks you thoroughly and you look like a drowned rat. So don’t worry about being labelled a hipster, it might actually make a change to being labelled a cyclist.

OK, I’m sold, but what it’s called again? Fixie or fixed?

The word fixie has become synonymous with the hipster craze for riding, er single speed bikes even though most do not actually ride fixed gear bikes. Walk into your local bike shop and ask for a fixie and you’ll be ushered towards the bright multi-coloured bikes with cheap components parked in front of the skinny fit three-quarter length jeans (and any other stereotype I can muster).
Ask for a fixed gear bike in a gruff voice and depending on the size of your thighs, you’ll either be taken towards the black track bikes with no brakes or over towards some nice looking fixed gear bikes with front brakes ready for road riding. Alternatively you could build your own fixed gear, which is definitely on my cycling bucket list one cold winter day.

What gears should I ride and what the heck is a flip-flop hub?

Steepest hill climb
Not a day for the fixed gear
A flip-flop hub is essentially a back wheel with two gears, one on either side of the wheel, allowing you to flip the wheel around and choose another gear. Handy eh? Now, don’t think of this as a mid-ride gear change, because flipping your wheel every time you hit a hill is inconvenient to say the least, especially for the bunch waiting for you at the top.
A flip-flop hub allows you to choose between two gears or alternatively between a freewheel and fixed gear. You can have a small cog for those fast flat rides when you’re focussing on strength and a big cog on the other side for when you just want to spin away like a hamster on a wheel. I tend to avoid extremes when choosing my gears although my flip-flop combination does have a broadish range (14 and 17 tooth) to ensure my ride is comfortable but very different whichever gear I choose.
Most people use a flip-flop hub to alternate between riding fixed and coasting along on a freewheel. Having a road bike with all of the free-wheeling gears I need, I don’t really need such freedom but can understand why the thought of riding fixed on tired legs might have me reaching for the spanner set.

What is the best gear ratio for fixed gear bikes

Massive bike gear
Pray for a tailwind
There is no universal optimum gear for riding fixed just like there’s no one best anything for anybody. Much will depend on four things: terrain, fitness, cadence and your desired average speed. You can search as many forums as you like to find the best gear ratio but the only way to find that magic formula is to experiment.
Use a gear calculator to begin with, which will help you understand what gears are required for certain speeds when riding different cadences. Finding the right gear is like finding the right woman or man. Many will seem attractive but it’s not until you’ve spent some time with them and er, ridden ’em until you’ll really know. Once the magic happens you will never flirt with another gearing ever again.
Me? Well, I’m in denial. I ride gears either side of what I think my special gear will be. I ride a 42 on the front simply because it came with the bike (in truth I’d prefer something a little bigger from a purely aesthetic view, along with a minor uplift in efficiency too – big front cogs transfer more power than small cogs).
On the back I ride either 14 or 17 depending on how I feel. The 14 is a little tough going when accelerating from a standing stop but I like to mash out some quick revolutions on this gear when feeling strong. The 17 is almost perfect for commuting but is a little too spinny, which helps ensure I stick to low tempo recovery rides, something I am incapable of doing on a geared bike. Some day I’ll splash out on that special something, which is probably a 16. In the meantime I’ll continue to enjoy the tease.
So off you go, deep into that murky world of gear ratios and gear inches and 44×16, 48×16 and 52×19 debates that will drive you crazy and leave others looking bemused. Before you know it you’ll begin researching skid patches, alienating even your closest of riding friends.
Ride fixed, ride free. Whatever you ride, smile.
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Tuesday, 20 March 2018

Role of Polysaccharides on Mechanical and Adhesion Properties of Flax Fibres in Flax/PLA Biocomposite

International Journal of Polymer Science
Volume 2011 (2011), Article ID 503940, 11 pages

LIMATB (Laboratoire d'Ingénierie des MATériaux de Bretagne), Centre de Recherche, Université de Bretagne Sud (UBS-Ueb), Rue de Saint Maudé, 56321 Lorient Cedex, France
Received 10 January 2011; Accepted 2 March 2011
Academic Editor: Susheel Kalia
Copyright © 2011 Gijo Raj et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The effect of alkali and enzymatic treatments on flax fibre morphology, mechanical, and adhesion properties was investigated. The multilength scale analysis allows for the correlation of the fibre's morphological changes induced by the treatments with mechanical properties to better explain the adherence properties between flax and PLA. The atomic force microscopy (AFM) images revealed the removal of primary layers, upon treatments, down to cellulose microfibrils present in the secondary layers. The variation in mechanical properties was found to be dependent, apart from the crystalline content, on interaction between cellulose microfibrils and encrusting polysaccharides, pectins and hemicelluloses, in the secondary layers. Finally, microbond tests between the modified fibres and PLA emphasize the important role of the outer fibre's surface on the overall composite properties. It was observed here that gentle treatments of the fibres, down to the oriented microfibrils, are favourable to a better adherence with a PLA drop. This paper highlights the important role of amorphous polymers, hemicellulose and pectin, in the optimisation of the adhesion and mechanical properties of flax fibres in the biocomposite.

1. Introduction

Research for environmental friendly alternatives has led the composite community to develop new “ecobiocomposites,” made from natural fibres and biodegradable polymer matrices, such as polylactic acid (PLA) [13]. It was reported that the specific Young’s modulus of PLA/Flax biocomposite ( GPa for 25% fibre volume fraction) can be as close to that of glass/polyester composites ( GPa) [1] and makes them suitable for interesting applications.
Even though natural fibres have ecofriendly credentials, they present some major drawbacks, such as poor thermal stability, anisotropic resistance, high moisture absorption heterogeneity, and in some cases poor incompatibility with polymer matrices [3]. These drawbacks prevent the use of natural fibre reinforcements in high performance structural composite applications and limit, up to now, their use for nonstructural parts. The complex chemical and physical structure of natural fibres [45] is certainly responsible for these limitations which may be overcome using different chemical or physical surface treatments.
A detailed description of the flax fibre structure can be found in the literature [45]. Briefly, a flax fibre consists of (i) a middle lamella region, principally made up of pectin, with small quantities of lignin that ensures bundle cohesion, (ii) a primary cell wall which forms ~10% of the fibre’s diameter and mainly consists of cellulose microfibrils embedded in a matrix of pectin, hemicelluloses, and small quantities of lignin [56], and (iii) a secondary cell wall, which makes up 90% of the cell cross section and mainly consists of three layers of cellulose microfibrils, with a mean axial orientation of 10°, bounded with pectin and hemicellulose. In this layer, pectin and hemicellulose are forming an interphase between cellulose microfibrils [57] and are thus called “encrusting polymers.” Cellulose microfibrils with typical diameters about 25–30 nm [4] consist of highly ordered crystalline cellulose zones (with typical Young’s modulus up to 143 Gpa [8]), in which cellulose crystals are arranged periodically and longitudinally along the fibre’s axis, interconnected by amorphous cellulose zones. An important issue concerns the hierarchical organisation and structuration of different polysaccharides in the different layers of the fibre and the structural difference between primary and secondary layers. This point is very important to better understand the mechanical and surface properties of flax fibre and consequently to better understand the effect of specific treatments to improve the mechanical properties of the fibre-reinforced composite.
During the past decade, numerous works have been published in the literature concerning modifications of flax fibres to improve the interface adhesion with a polymeric matrix [910]. Among the different chemical treatments, alkali treatment of flax fibres was found to be simple and efficient in improving the adhesion with polymer matrices [1112]. This treatment is known to remove amorphous polysaccharides such as hemicellulose, pectin [5], lignin [13], and separate fibre bundles into elementary fibres. Additionally, alkaline solution reacts not only with noncellulosic materials, but also with cellulosic components creating large cellulose lattices, which can be converted to new crystalline structures (cellulose II) [12]. Another effect of mercerisation is the possible depolymerisation of the native cellulose type I molecular structure into short-length crystallites [14]. This last effect has a direct negative effect on the fibre strength and stiffness [13].
Recently, enzymatic treatments involving the use of mainly pectinase enzymes have gained the attention of the scientific community as an environmental friendly treatment on flax fibres [15]. These treatments were found to be efficient in removing the shive and epidermal tissues of fibres, as characterized by optical and Scanning Electron Microscopy studies [16]. Commercially available enzyme preparations mainly contain pectinase, as well as some quantities of hemicellulase and cellulase enzymes. Enzymes rich in pectinase and poor in cellulase are generally more suited to avoid any cellulose alteration within the fibre. However, it has been shown in a recent work, using X-ray scattering techniques, that cellulase enzymes act only on the surface of cellulose microfibrils and are not able to penetrate into nanopores of the cellulose crystallites without affecting the degree of crystallinity [17].
Although modifications of flax fibres by both chemical and enzymatic methods result in improved mechanical properties of the biocomposite, a detailed investigation of the relation between the morphological changes of the fibre and the resulting properties, at different length scales, is necessary to better understand the complex properties of such heterogeneous fibres. Generally, the overall properties of the composites are probed by macroscopic and indirect analysis such as mechanical tests [1118], water sorption studies [19], or chemical analysis [2021]. A major limitation of such macroscopic studies comes from the fact that they do not give a direct and clear relationship between the structural modifications of the fibre and their consequence on the fibre and composite properties. Recently, nanoscale studies of flax fibre using atomic force microscopy have emphasized the relevance of a multiscale investigation on such materials and provide valuable morphological and quantitative data to characterize the fibre’s surface properties [2223]. These works were complementary to traditional determination of interfacial energies using contact angle measurements, which is difficult to experimentally perform on heterogeneous samples such as that of flax fibres.
The aim of this paper is to highlight the significant factors acting on the interfacial adherence in biocomposites. The morphology of alkali- and enzymatic-treated fibres is investigated at different length scales, in order to better understand the relationship between the structure and the mechanical properties of the flax fibre. The adhesion properties at the fibre-polymer matrix interface are followed by microbond pull-out tests and force-volume adhesion mapping and discussed in the framework of the morphological and chemical modification induced by the treatments.

2. Materials and Methods

2.1. Alkali Treatment of the Flax Fibres
Dew-retted flax fibres (variety Hermes) grown in the Normandy region (France) were used in this study. Flax fibres were initially soaked in NaOH solutions (1%, 3%, 5%, and 10%) for 20 minutes at 23°C. The fibres were filtered and washed thoroughly in Milli-Q water before being rinsed in a very diluted solution of HCl (0.01 M) to remove excessive NaOH [12] and washed again with Milli-Q water before drying in vacuum at 65°C for 3 hours.
2.2. Enzyme Treatment of Flax Fibres
Enzyme treatments of flax fibres were carried out using a pectinase enzyme, made from Aspergillus aculeatus, Pectinex Ultra SPL (activity 9500 units/mL, Sigma Aldrich). The enzyme solution was diluted to a concentration of 20% in an acetate buffer solution (Sigma Aldrich) having a pH of 4.6 [22]. About 1 g of flax fibres was treated in 100 mL of the 20% enzymatic solution at 40°C. Fibres were then taken out of the treatment at intervals of 5 H and 18 H, washed at least 5 times in Milli-Q water, and dried in vacuum at 65°C for 5 hours before any analysis.
2.3. Scanning Electron Microscopy (SEM)
Morphology of raw-, alkali-, and enzyme-treated flax fibres was examined using a Jeol JSM 6460 LV Scanning electron microscope. Flax fibre samples were metallised (Edwards Scancoat six metallizer) for 10 minutes prior to SEM imaging.
2.4. Atomic Force Microscopy (AFM)
AFM experiments were conducted using a commercial Multimode Nanoscope IIIa atomic force microscope (Veeco, USA). Images were acquired in tapping mode (TM-AFM) under ambient conditions (23°C and RH 56%) using silicon tips (LTESP, Veeco). Samples were prepared by gluing an elementary flax fibre by their two extremities on a magnetic steel disc in order to keep it fixed during image acquisition.
Force volume (FV) mode was used to construct an adhesion force map of the various fibres. In FV mode, a contact mode image of the fibre’s surface was first made using a conventional contact mode tip (spring constant 0.57 N/m). Force maps were captured in the “relative trigger” mode to ensure that the maximum loaded force exerted on the sample is the same in every force plot. The maximum cantilever deflection was fixed at 160 nm (91 nN). The image XY resolution parameter, sample/line, was kept at 128, and the number of force/line was 64. Thus, for a scan line of 1.6 μm, force plots were recorded at every 25 nm.
2.5. Infrared (IR) Spectroscopy
Infrared (ATR-IR) spectroscopy (IR Perkin Elmer spectrometer) was performed on raw-, NaOH-, and enzyme-treated flax fibres using the attenuated total reflectance method with a mountable unit (Golden Gate). Spectra were acquired with an accumulation of 25 scans and were recorded in the transmittance mode in the range 400–4000 cm−1.
2.6. Single Fibre Tension Test (SFTT)
Flax single fibres were manually extracted and glued at both ends onto a piece of paper which was already punched to make a 10 mm hole (equal to the initial length L0 of the fibre). The mean diameter of flax fibres was determined by optical microscopy and obtained by analysing at least 15 different zones on each fibre. A minimum of fifty fibres, for each sample (raw-, NaOH-, and enzymatic-treated fibres), were analysed. The longitudinal Young’s modulus of the flax fibres was determined from the tensile loading of elementary flax fibres respecting the standard methods (NFT 25-704, ASTM 3379-75), that also takes into account the compliance of the system. The tension experiments were performed in a tensile testing machine (MTS Synergie 1000) equipped with a load cell capture that allows for measurements in the range of 0–2 N with an accuracy of 0.01%. Loading rate was kept constant at 1 mm/min throughout all experiments.
2.7. Wide-Angle X-Ray Scattering (WAXS)
X-ray scattering experiments were performed on a custom-built SAXS/WAXS machine equipped with a Rigaku MicroMax-007 HF rotating anode generator (Å). The size of the point-like X-ray beam on the sample was approximately. 300 μm. The 2D WAXS data were collected using X-ray sensitive Fuji image plates with a pixel size of  μm2. The modulus of the scattering vector  (, where θ is the Bragg angle) was calibrated using three diffraction orders of silver behenate. The data reduction and analysis including geometrical and background correction, visualization, and radial integration of the 2D diffractograms were performed using home-built routines designed using the IgorPro software package.
2.8. Microbond Test
The adherence strength between the flax fibres and the PLA matrix (Biomer L9000) was estimated by calculating the apparent interfacial shear stress (IFSS) values obtained from microbond tests on a minimum of ten pull-out experiments. A homogeneous PLA polymer microdroplet (<200 μm) was deposited on the surface of the flax fibre. In order to achieve this microdroplet, a microknot was made on the flax fibre with a microfilament of PLA. The setup was placed in an oven preheated at 190°C for 10 minutes. The sample was then immediately taken from the oven and quenched at room temperature. Prior to any tests, the diameter of the fibre near the PLA droplet, the embedded length of the fibre, and the drop height were measured using an optical microscope. Pull-out experiments were performed on a tensile testing machine (MTS Synergie 1000, load cell 2N). On the lower clamp, a homemade X-Y translator with two sharp knife edges was mounted. The microdroplet was brought just under these knife edges, and the knife blades were brought close together so that the blades just touch the upper end of the droplet [21]. Tensile loading was applied at the rate of 0.1 mm/min.

3. Results and Discussion

3.1. Morphology of the Flax Fibres
SEM images of raw and treated bundles (Figures 1(a)1(b), and 1(c)) provide a macroscopic investigation of the morphology of the outer surface of the fibres. Figure 1(a) shows the bundle structure of raw flax fibres formed by elementary fibres held together mainly by pectin, lignin, and amorphous polymers found in the primary cell wall and in the middle lamellae region [5]. On the other hand, the two modified fibres, following NaOH and enzymatic treatment (Figures 1(b) and 1(c), resp.), appeared to be well separated.
Figure 1: SEM images of flax fibres: (a) raw flax bundle, (b) 10% NaOH-treated bundle, (c) 18 H Enzyme-treated bundle, (d) raw elementary fibre, (e) 10% NaOH-treated elementary fibre, and (f) 18 H enzyme-treated elementary fibre.
Figures 1(d)1(e), and 1(f) show SEM images of elementary fibres of raw-, NaOH-, and enzyme-treated flax respectively. The surface of the treated fibres (Figures 1(e) and 1(f)) appears free of any residual particles, homogeneous and smoother, which reveal the efficiency of both treatments in removing some large particles and entities present on the raw fibre (Figure 1(d)). This result contrasts with some earlier studies of the effect of NaOH on the structure of natural fibres, which reported a drastic change of the fibre morphology that led to a decrease of the fibre diameter and the development of a rough and irregular surface [24]. It should be mentioned that in some cases, the NaOH treatment was more severe (higher concentration) [13] and less controlled (time and temperature) [21] than in more recent studies [12].
Atomic force microscopy was used to observe the surface of the flax fibres with a nanometer resolution. Tapping mode (TM) AFM images reveal similar trends as those observed by SEM (Figure 2). Large globular entities or particles that envelop the fibre’s surface can be observed on raw fibres (Figure 2(a)). Upon treatment, these particles disappeared, and the flax fibre’s surface becomes relatively smooth. The alkali treatment yields a cleaning effect of the fibre’s surface. For a concentration of 5%, the AFM image reveals an unidirectional orientation of microfibrils in the secondary layer (Figure 2(b)). At the highest NaOH concentration (10%), a better resolution of the oriented microfibrils is obtained, with typical diameters between 20 and 40 nm (Figure 2(c)). These oriented fibrils are all aligned within a similar direction to the main fibre axis. These entities correspond well to the description of crystalline cellulosic microfibrils within the secondary cell wall of flax fibre, both in terms of size and orientation [4].
Figure 2: TM-AFM phase images (scan size of 4 μm²) of (a) raw-, (b) 5% NaOH-, and (c) 10% NaOH-treated flax fibres.
The pectinase enzyme preparation, which contains pectolytic and a range of hemicellulolytic activities, has the ability to depolymerise major components of plant cell walls. After 5 H of enzymatic treatment, the flax fibre still presents some large inhomogeneous and rough areas which seem to cover, like an envelope, a more organised layer made of aligned structures (Figure 3(a)). After 18 H of treatment, a more uniform surface can be observed, showing only oriented microfibrils (Figure 3(b)). A high resolution 3D image of enzyme-treated (18 H) flax fibre (Figure 3(c)) allows for the visualisation of the oriented cellulose microfibrils (in the size range 25 to 30 nm) and indicates the preferred orientation along the fibre longitudinal axis (Figure 3(d)). In Figure 3(d), the axis of the fibre is indicated by the arrow that goes through the maximum heights of the fibre, and we can qualitatively observe that the microfibril angle is somehow quite close to the main fibre axis [4].
Figure 3: TM-AFM phase images (scan size of 4 μm²) of enzyme-treated fibres of (a) 5 hours, and (b) 18 hours, (c) TM-AFM 3D phase image of the microfibrils showing their preferential orientation (scan size: 1 μm²), (d) TM-AFM 3D height profile showing the main fibre axis going through the maximum height of the image.
The multiscale complementary approach, using SEM and AFM, confirms the removal of some amorphous polysaccharides from the outermost surface of the flax fibres (middle and primary layers) to provide a clean and smooth surface with well-aligned cellulose microfibrils with an orientation angle close to the fibril’s axis, typical of the secondary layer.
Surface chemical modifications induced by the treatments were investigated by ATR-IR spectroscopy, and the recorded spectra are displayed in Figure 4. When compared to raw flax fibre, ATR-IR spectra of treated fibres show a decrease of intensity of the peaks at 1615 cm−1 (corresponding to pectin [5]), and at 2923 cm−1 (C–H stretching vibration in hemicellulose [25]). Moreover, a shoulder peak at 1735 cm−1, that corresponds to “C=O” stretching of carboxylic acid or ester group of hemicelluloses [26], disappeared after the treatments. These observations show that pectin and hemicelluloses are the main compounds that are removed following the treatments. However, since the penetration depth of the ATR-IR evanescent wave is several microns, we cannot distinguish the origin of these polymers, either coming from the primary cell wall or from the secondary one.
Figure 4: ATR-IR spectroscopy of raw-, NaOH-, and Enzyme-treated flax fibres: (a) peak at 1615 cm−1 that corresponds to pectin; (b), (c) peaks at 1735 cm−1 and 2923 cm−1, respectively, that correspond to hemicellulose; (d) peak at 1440 cm−1 that corresponds to lignin.
3.2. Mechanical Properties of Flax Fibre
The tensile mechanical properties of the raw and the treated flax fibres were evaluated by the single fibre tension test (SFTT) experiment. Figure 5 represents typical stress-strain curves of different elementary flax fibres. From the raw fibre’s traction curve, two distinct regions can be identified. First, a nonlinear part, from 0 to ~350 MPa, is observed during low deformation at the initial stages of the loading curve. This first part can be associated with the global loading of the fibre, through the deformation of each cell wall structure [27] including a sliding of the microfibrils along their progressive alignment with the main fibre axis and a reorganisation of the amorphous matrix (mainly pectins and hemicelluloses) surrounding the microfibrils. The second region of the loading curve appears linear, characteristic of an elastic deformation, and corresponds to the response of the aligned microfibrils to the applied tensile strain. After reaching a maximum value of tensile stress, the fibre breaks. From the slope of the linear part of stress versus strain curve, one can extract a “global” (average) longitudinal Young’s modulus of the fibre. The calculated Young’s modulus for raw flax fibres from these experiments was found to be  GPa (Table 1), which corresponds well with reported data [427]. Taking into account the internal composite structure of flax fibres, any discontinuity that may appear in the linear part of the stress versus strain can give information regarding any internal structural changes of the different layers of the fibres during loading.
Table 1: Comparison of Young’s modulus () and tensile stress (σ) of raw-, NaOH-, and enzyme-treated flax fibres measured by single fibre tension test.
Figure 5: Comparison of stress versus strain curves of raw-, NaOH-, and Enzyme-treated elementary flax fibres under tensile loading. Shaded region corresponds to a threshold stress level bellow which fibres broke before pullout in microbonding test.
Stress-strain curves obtained for alkali- and enzymatic-treated flax fibres are significantly different from that of the raw flax fibre. For the 1% NaOH-treated fibres, the initial nonlinear part is still visible, but as the concentration of the alkali increases, the initial “nonlinear” part of the stress-strain graph disappears to give a straight and linear curve. As revealed by AFM analysis, observation of the organised and oriented microfibrils was only possible for samples treated with a 5% NaOH solution. For NaOH concentrations of 5% and 10%, the stress-strain curves present two linear regions with a transition that evolves with the increase of concentration of the sodium hydroxide solutions (up to ~400 MPa for the 5% NaOH-treated fibre, and up to 200 MPa for the 10%-treated one). This typical nonelastic behaviour can be related to the internal structural changes in the fibre, such as swelling, induced by the alkali treatment. The curves resemble a biphasic stress-strain curve which is typically observed in tensile loading of wood fibres [28], in which the first slope of the curve is due to the orientation of microfibrils in the direction of deformation. After a yield point, corresponding to a threshold stress, a second linear stage with a smaller slope evolved and can be interpreted as a consequence of plastic deformation of the polysaccharide matrix accompanied by a slippage mechanism of microfibrils under tensile loading [28]. Keckes et al. [29] proposed a simple “molecular Velcro” model to explain this deformation process in which a large number of molecular bonds, due to entangled hemicellulose chains, may disrupt upon stress transfer between cellulose fibrils and amorphous matrix.
In the case of these two NaOH treatments, traction curves observed here are likely obtained from the response of the secondary layer of the fibre, as observed by AFM. Morvan et al. [5] described the organisation of this layer and proposed a close relation between the two encrusting amorphous polymers, pectins and hemicelluloses, and crystalline cellulose microfibrils. In this system, molecular bonds between pectin and hemicellulose, including hydrogen van der Waals interactions, may break upon loading in order to dissipate the stress and could explain the “pseudoplastic” deformation observed at 10% of NaOH. Further, a linearly increasing slope after the deformation region can be attributed to the rearrangement of cellulose fibrils in the direction of strain [30]. Following this “severe” treatment, the NaOH solution may have attacked irreversibly the encrusting hemicellulose and pectin macromolecules in the secondary layer and thus weaken their ability to efficiently transfer the stress between adjacent cellulose microfibrils. The decreasing value of yield stress of fibres with increasing concentration of alkali (above 5% NaOH) is a clear indication of the severity of alkali attack on encrusting polymers, which should be avoided to conserve the good mechanical properties of the fibre. Another important feature concerns the stress at break value obtained for the different fibres. We can observe a decrease from 1100 MPa for the raw fibre to ~550 MPa, as the concentration of NaOH treatment was increased up to 10%. Wide-angle X-ray diffraction experiments, performed on raw- and alkali-treated fibres, do not show any significant difference (Figure 6) and are typical of microcrystalline cellulose [17]. Reflection peaks at °, 16.4°, and 14.8° that correspond to native cellulose I were found, as reported in the literature [3132]. WAXS experiments indicate that the alkali treatment conditions used in this study did not alter the crystalline part of the fibre and confirm that a polymorphic transition of cellulose I structure to cellulose II did not occur. The decrease in the stress at break for alkali-treated flax fibres can thus be preferentially attributed to an alteration of amorphous noncellulosic polysaccharides, pectins and hemicelluloses, present in the primary and secondary cell wall layers of the fibre. Furthermore, the presence of natural defects found in flax fibres, such as kink bands, may also play an important role in the decrease of the stress since they may become more brittle after treatments, due to a favourable and rapid diffusion of the alkali solution through these defects and down to the internal structure of the fibre. It has been shown earlier that, under tensile loading, the fibre starts to break at the region near the kink band where the crack initiates [33]. It is worth noting that for alkali-treated fibres, as the concentration of NaOH increases, the area under the stress-strain curve increases considerably. This is an important property change since the total area under the curve corresponds to the energy per unit volume absorbed in the fibre until its failure and thus to a measure of toughness of the material [28] which is interesting for some applications like energy absorption or vibration damping. Our experiments show that by controlling the NaOH treatment conditions, the toughness of the fibres can be tuned for specific applications.
Figure 6: WAXS pattern of raw and different NaOH-treated flax fibres.
In the case of pectinase enzyme-treated flax fibres, after 5 hrs of treatment, the shape of the stress-stain curve was similar to that of raw flax fibres (Figure 6). It showed an initial nonlinear part followed by an increasing linear part corresponding to an elastic deformation until the fibre breaks. The nonlinear initial part of the stress-strain graph indicates the presence of a significant amount of residual amorphous polymers from the primary cell wall, as observed by the AFM images. On the other hand, flax fibres treated for 18 hours show only a single linear slope that extends until the fibre breaks, which is representative of a direct elastic response of the microfibrils under loading, in good correlation with AFM observations. Contrary to NaOH-treated flax fibres, the enzyme-treated fibres did not show biphasic stress-strain curves, even after prolonged treatment times (18 H). This result can be explained by the specificity of the pectinase enzyme treatment that largely acts on pectin and do not induces any swelling of the material. It is thus likely that encrusting hemicelluloses are not altered by the pectinase treatment and thus maintained the cohesion of the crystalline cellulose microfibrils within the secondary layer. This is an important result which shows that a specific enzymatic treatment is more suitable to keep the intrinsic stiffness of the flax fibres and emphasizes the important role of the encrusting amorphous hemicellulose in the secondary layer to maintain the cohesion between cellulose microfibrils. However, it should be also noted that the stress at break of 18 H enzyme-treated fibres was comparatively less than the 5 H-treated fibres (Table 1). This behaviour can be associated with a possible alteration of defects such as kink bands within the fibre upon a prolonged treatment.
3.3. Adherence/Adhesion Properties
As previously discussed, another important issue in the development of biocomposites is the optimisation of the interface in order to promote a good stress transfer between the fibre and the polymer matrix. At the macroscopic scale, the microbonding pull-out test is well suited to directly determine the adherence between a reinforcement fibre and the polymer matrix [34] and was thus adapted to directly measure the adherence between an elementary flax fibre and a polylactic acid polymer drop. The apparent interfacial shear stress (IFSS), , was calculated using the following Kelly-Tyson [35] equation:where  is the maximum pull-off force at debonding,  is the fibre diameter, and  is the fibre-embedded length in the polymer droplet. The above equation assumes that the force  at the instant of debonding is predicted to be directly proportional to the joined surface area between the fibre and matrix, and the droplet shears off from the fibre surface when the average shear stress at the interface, , becomes large enough to break the interface. The apparent shear stress was determined from the linear regression of the plot of debonding force versus bonding area. The apparent interfacial shear stress values (IFSS), , calculated for PLA matrix and different flax fibres are summarized in Table 2. We observed that apparent IFSS values improved after both treatments. In both cases, the maximum IFSS values were obtained for the mild treatment conditions, that is, at 1% of NaOH and after 5 H of enzymatic treatment. However, for more aggressive treatment conditions, that is, at 5% NaOH, 10% NaOH, and 18 H enzyme treatments, IFSS values were not validated since the fibres break cohesively before the complete pull-out event during microbond testing. Earlier, the elastic properties of these fibres (Figure 5) were shown to be greatly reduced when the treatments reached the secondary layers and attacked the encrusting polymers. It was observed that successful fibre pull-out tests were obtained for fibres with tensile strength values above a threshold stress level of ~800 MPa. Similar observations were already reported by Pommet et al. [36] on bacterial cellulose-modified sisal fibres/PLA system where cohesive failure of the fibre occurred when interfacial adhesion exceeded the adhesion among the different constituents of the fibre.
Table 2: Interfacial shear stress values of raw-, NaOH-, and enzyme-treated flax fibres calculated from microbonding test with a PLA droplet.
An improved adhesion between modified fibre and the PLA polymer can be developed through different mechanisms. For example, Pommet et al. [36] suggested that the presence of cellulose microfibrils increases the roughness of the fibre’s surface, that may enhance the adhesion through mechanical interlocking mechanisms. Other explanations such as the presence of hydrogen bonds between hydroxyl groups present on cellulose fibrils and carbonyl groups in PLA were also argumented [36]. The hypothesis of the fibre’s roughness is not validated here since the fibre becomes smoother after the treatment, as revealed by AFM; however, we expect some strong interactions between cellulose fibrils and PLA polymer when some weakly adhering polysaccharides are removed from the fibres, through, for example, hydrogen and van der Waals interactions [37]. However, the pull-out test cannot directly distinguish the nature of these different adhesion interactions.
In a recent work [38], we demonstrate that the force volume technique provides valuable information on the adhesion properties of heterogeneous natural fibres at the nanoscopic scale. Force volume displays simultaneously a topographic image of the fibre surface and the corresponding adhesion map between the fibre surface and a standard silicon nitride AFM tip. The adhesion mapping is obtained from the measurement of force-distance curves on all the different coordinates of the fibres’ scanned area (Figure 7). The topographic height image of the sample is crucial in the interpretation of adhesion force maps, since the adhesion interaction between the tip and the sample is dependent on the contact area between the tip and sample. For heterogeneous flax samples, the technique allows for a semiquantitative comparison of the changes in the topography and adhesion properties of raw and different treated fibres.
Figure 7: AFM Force-volume images: topography, FV adhesion force map, and force plots corresponding to raw flax fibres ((a), (b), and (c), resp.); NaOH-treated fibres ((d), (e), and (f), resp.); enzyme-treated flax ((g), (h), and (i), resp.).
The adhesion map of raw flax fibres (Figure 7(b)) reveals a heterogeneous distribution of adhesion forces across the fibre surface. Some large aggregates can be found on the outer surface of the raw fibre, as already discussed in the first part. High values of adhesion forces were recorded on such compounds, as indicated by darker regions in the adhesion map image. Average adhesion forces measured on raw flax fibre surface ranged from 41 to 73 nN. FV images of alkali-(Figure 7(e)) and enzymatic-treated fibres (Figure 7(h)) contrast well with those of raw flax fibres. In the latter case, a more homogeneous fibre surface can be observed. The adhesion peaks of the force plots are all superposed and present a small adhesion with the AFM tip. The adhesion forces measured on the treated fibres ranged from 6.5 to 10 nN.
In the absence of electrostatic and chemical bonding forces, adhesion between an AFM tip and the fibre’s surface results mainly from van der Waals interactions and capillary forces that arise from condensation of water molecules, present in ambient conditions (56% of relative humidity), between the tip and the surface [39]. Thus, under ambient humidity, adhesion forces are largely influenced by capillary forces, which in turn are dependent on the hydrophilic/hydrophobic properties of the surface and the AFM tip. The latter is known to be hydrophilic. Generally, for hydrophilic surfaces, increased relative humidity leads to an increase of the capillary forces [40]. On the other hand, if the surface is hydrophobic in nature, the contribution from the capillary forces on the adhesion force measurements is less and can even be independent of relative humidity [41]. Thus, we presume that high adhesion forces measured on raw flax fibres are mainly due to the presence of pectin materials that is known to be the more hydrophilic polysaccharide [42] in the fibre and present in the middle lamellae and the primary layer. When this polysaccharide is removed from the fibre’s surface as a consequence of the treatment, as revealed by AFM, the outer surface is mainly constituted by cellulose microfibrils, which in turn gives to the fibre different chemical properties. Biermann et al. [43] studied the chemical nature of crystalline cellulose surface by dynamic molecular simulations and reported a nonhydrophilic nature. Another recent study by Zykwinska et al. [44] reported that the (1 0 0) crystal plane of cellulose is hydrophobic since CH groups are exposed at the surface, whereas the OH groups are reported to form specific types of hydrogen bonds within the crystalline regions [45]. These findings suggest that crystalline regions in cellulose are rather hydrophobic in regards of the amorphous polysaccharides which can present free hydroxyl groups and consequently may form hydrogen bonds with water molecules on the surface. This hypothesis is in good agreement with the lower adhesion forces measured on treated fibres. These results are also well correlated with the microbond test. The low adhesion forces measured by force volume are synonymous of a surface that is less hydrophilic, which may lead to a higher adhesion with the rather hydrophobic PLA matrix.

4. Conclusion

The effect of alkali and enzymatic treatments on the fibre’s structure, surface composition, and adhesion properties was investigated. A major consequence of these treatments is the removal of some weakly adhering amorphous polysaccharides, mainly pectins and hemicelluloses, essentially from the middle lamellae, primary cell wall and possibly from the secondary cell wall, as observed by AFM. The organisation of these polymers within these layers was found to have a profound impact on the overall mechanical properties of the fibre, as revealed by traction tests on elementary fibres. We demonstrate that pectins and hemicelluloses, in the primary layer, do not significantly impact the mechanical properties of the fibre, whereas more pronounced treatments, that reach the secondary layer and attack encrusting polymers, may decrease the strength of the fibre, by reducing the interactions between amorphous and crystalline polymers. In parallel, adhesion properties of the treated fibres were examined at different scales and proved to be essential in the optimisation of a composite system. Microbond tests reveal that PLA adhesion on the flax secondary layer with oriented cellulose microfibrils was found to be the most important, through different mechanisms, mainly van der Waals interactions and H bonding. For an optimal performance of biocomposites, an improvement in interface adhesion with PLA while preserving flax fibre intrinsic mechanical properties was achieved here at gentle alkali treatments (1%, for 20 minutes) and more safely by enzymatic treatments (up to 5 Hrs). The latter ecofriendly treatment proves that this process is becoming highly attractive in the biocomposite industry.


Financial support for this work from Region Bretagne, Cap Lorient, and the Morbihan Department (France) is deeply acknowledged. The Authors thank Professor Dimitri A. Ivanov, ICSI, Mulhouse (France) for performing WAXS experiments on flax fibres.


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