Dr Hector G Lopez-Ruiz
email: hector.lopez-ruiz@let.ish-lyon.cnrs.fr

Prof Yves Crozet
email: yves.crozet@let.ish-lyon.cnrs.fr

SUSTAINABLE TRANSPORT IN FRANCE: IS A 75% REDUCTION IN CO 2 EMISSIONS ATTAINABLE?

Keywords: Greenhouse gas, long-term, scenario, transport, sustainable development

Today, numerous works conclude that transport seems to be completely coupled to economic and export/import growth. Therefore, as a direct consequence of economic development transport sits today as one of the major final energy consumers and one of the most important sources of carbon dioxide emissions. Consequently, this situation of continuous increase in transport clearly poses an environmental problem. In this paper we propose to asses a certain number of possible solutions through scenario building in a backcasting manner using the TILT (Transport Issues in the Long Term) model. In particular, we evaluate three different scenarios that address how technology and different public policies can contribute towards a sharp reduction in CO 2 emissions. Furthermore, we propose an estimation of infrastructure investment needs as well as insight on how transport budgets (time and monetary) could evolve in each of the three scenarios presented:

 Pegasus -promoting strict technology standards  Chronos -promoting green multimodality  Hestia -promoting transport-GDP growth decoupling.

Each scenario allows a quick comprehension of the types of results that can be obtained through different policy mixes. In sum, realistic technological hypothesis show that a 50% reduction in emissions, from the 2000 level, is a clear possibility, and that the remaining 25 % reduction in emissions is possible through different types of policy mixes.

Introduction

Today, numerous works conclude that transport seems to be completely coupled to economic growth. Therefore, as a direct consequence of economic development, transport sits today as one of the major final energy consumers and one of the most important sources of carbon dioxide emissions. Furthermore, in the absence of major technological change, this unsustainable situation will most undoubtedly get worse in the future. Consequently, this situation of continuous increase in freight and passenger mobility clearly poses an environmental problem in a world that is trying to attain sustainability.

It is in part from this preoccupation that spawned the Kyoto Protocol, ratified by the European Union member countries, which came into effect on February 16 2005 and which was a first vital step towards further global emission reductions.

In 2005, the Kyoto protocol signees committed to an average reduction of 2.8% of greenhouse gas emissions by 2008-2012 (from the 1990 level). This goal would require concentrations of CO 2 well below 550 parts per million (ppm) CO 2 -equivalent.

In order to have a reasonable chance of keeping concentrations well below 550 ppm CO 2 -equivalent, analyses for the EU Environment Council show that global emissions should be limited to an increase of 35 % above the 1990 level by 2020 and then decrease to 15 % below the 1990 level by 2050. However, to reduce the risk of overshooting the 2°C target, recent scientific insight has shown that it is possible that a 50% reduction in global emissions will be necessary for 2050. This has pushed the EU Environment Council to adopt the conclusion that, in order to achieve stabilization, developed countries should reduce emissions by about 15% to 30 % by 2020 and 60% to 80 % by 2050, below the base year levels (1990).

In this state of affairs, and in view of the uncertainty concerning the ultimate goal, French authorities have fixed themselves a 75% reduction objective from the 2000 level to be attained by 2050. This objective will serve as the work basis for our scenarios which will only focus on transport activities. In France, transport is at the origin of 25% of energy consumption and is responsible for at least, 30% of carbon dioxide emissions (80% of these emissions come from road transport).

Even tough we have seen the birth of new technologies in the transport sector and that we have witnessed a growing "social and entrepreneurial conscience", these environmentally friendly products and/or services have observed a very slow implementation. As A. SCHEAFER [START_REF] Schafer | Transportation in a Climate-Constrained World[END_REF] et al or D. SPERLING & N. LUTSEY [START_REF] Sperling | Energy efficiency in Passenger Transportation, The Bridge[END_REF] clearly underline in their work, technological progress cannot be effective if it is not accompanied by a deep change in organization and behavior, especially if we are aiming at a very important reduction of CO 2 like the 75% reduction target that French authorities have set.

Following this line of reasoning, the reduction of CO 2 emissions implies not only the need for new technologies and their widespread use but also an increased match in current technology supply and consumer demand through the use of incentive economic instruments. Thus, this reduction objective implies the need to set up a certain number of public policies ranging from inciting technological progress, to tolls, to intermodality/comodality development or even rationing (tradable emission permits).

In consequence, since we deemed necessary to explore the different options available, three sustainable transport scenarios (for both freight and passengers) were developed with the LET-ENERDATA group in 2008 [START_REF] Crozet | Comment satisfaire les objectifs internationaux de la France en termes d'émissions de gaz à effet de serre et de pollution transfrontières ? Programme de recherche consacré à la construction de scénarios de mobilité durable[END_REF]. This paper has two aims: Firstly, to present the main results for the French scenarios developed in 2008, which show that organizational policies can be mixed with technological advances and used to attain significant carbon reductions in the future. Secondly, to present new quantitative insight for each of the scenarios concerning public investment needs in infrastructure; public policy impact and the implications on transport monetary budgets [START_REF] Lopez-Ruiz H | Environnement & Mobilité 2050: des scenarios pour le facteur 4 (-75% de CO 2 en[END_REF].

As it will be presented, the TILT (Transport Issues in the Long Term) model was used to build three representative scenarios that depict viable (though strong) public action options that will sharply reduce CO 2 emissions. This same model was also used to quantify the effects of climate oriented policies on the transport system and the economy. The inherent logic in building these technico-organizational scenarios is linked to the idea of growing constraints -ranging from promoting new motor technologies to public policies aiming at decoupling transport activities and GDP. This underlying principle enables us to present three different scenarios that allow a quick comprehension of what can be obtained through policy mixes. These scenarios are: Pegasus, Chronos and Hestia, and are fully described in Section 3.2.

These three scenarios will be analyzed in order to come up with an assessment pertaining to the impact of four basic policy packages (that are then divided into ten different public policies) inspired by the work done in the VIBAT project [START_REF] Banister | Looking over the Horizon: Visioning and Backasting[END_REF]:

 behavior oriented policies  market oriented policies  regulation oriented policies  spatial and infrastructure planning policies.

The TILT Model (Transport Issues in the Long-Term)

In light of the environmental situation, it is, more than ever, necessary to shed some light on the important role played by transport activities in greenhouse gas emissions. It is equally important to develop environmental policies for reducing CO 2 emissions and to be able to model them correctly whilst assessing the effects of the different technological, institutional, regulatory and economic options available. The TILT model is a flexible tool that does just that. Furthermore, because sustainable development is a highly complex problem area (6 & 7) -which will probably call for major changes of industrialized societies and long-term strategic planning-the choice of method in order to partake our analytical study was very important. This is why, once the specific needs of the research project had been taken into consideration, a backcasting model for scenario building seemed to be the most appropriate way of proceeding.

Backcasting scenario building typically aims at providing policy makers (and an interested general public) with images of the future as a background for opinion-forming and decision-making [START_REF]Backcasting as a Tool in Competitive Analysis[END_REF]. Consequently, the results offer a new concrete and applied vision of the policies to partake, in order to attain an environmentally sustainable transport system. Moreover, in the field of future-oriented studies, the traditional forecasting approach is still dominant, but scenarios have come widely into use during past decades (9) because they allow a broader analysis when compared with a formalized prognosis methodology.

The basis of the TILT approach lies on the fact that it is based on a twofold structure composed of a macro-and a microeconomic part (see figure 1). This structure serves as the core transport model that will provide input to three additional modules: These modules are designed to give insight concerning: total energy use, CO 2 emissions, public policy sensitivity and economic impact for any specified scenario.

The core transport model is composed by a macroeconomic determination and a microeconomic optimization that need to be at equilibrium in order to obtain a coherent modal split as a result. Thus, scenario building using a backcasting methodology with the TILT model requires first, the specification of the desired future and second, a "back and forth" movement where there is an interaction between the macro-and the microeconomic modules in order to identify the different equilibriums possible that allow the attainment of the specified future. From these possible equilibriums, the LET-ENERDATA group chose three that best depict the range of solutions available. These three scenarios are:  Pegasus -promoting strict technology standards  Chronos -promoting green multimodality  Hestia -promoting the decoupling of transport activities and economic growth (GDP).

The macroeconomic determination of the TILT model

The macroeconomic component of the TILT (3) model is based on the VLEEM model (Very Long term Environment Energy Model), developed in the framework of a European project in 2002 [START_REF]VLEEM 2[END_REF]. The VLEEM model is based on a re-foundation of the energy-environment modeling structures in order to properly assess very long-term modifications of demographics as well as social and cultural preferences in relation to transport needs.

The macroeconomic component of the TILT model relies on a structure where population growth (determined by exogenous birth/death rates) and the evolution of population education levels (user determined) influence productivity in the different defined age classes. These changes in productivity, joined to demographic evolution, determine GDP growth as well as consumption levels and time use (work, sleep, self accomplishment, leisure and transport).

From the determination of time used in transport activities, the macroeconomic mobility determination is established through the use of average modal speeds that evolve based on a speed/GDP elasticity (which, in the case of France, has proven to be fairly stable over time [START_REF] Crozet | Comment satisfaire les objectifs internationaux de la France en termes d'émissions de gaz à effet de serre et de pollution transfrontières ? Programme de recherche consacré à la construction de scénarios de mobilité durable[END_REF]). This implies different modal split possibilities, given that the "need for speed" is sensitive to the affluence and freight value [START_REF] Schafer | The future mobility of the world population[END_REF]. Consequently, transport modal saturation rhythms can be varied -in the macroeconomic model-through public policies affecting speed/GDP elasticities. In order to have a more precise view of the effects of public policies on each scenario, a microeconomic component was developed within the TILT framework (4). This component allows further analysis of demand determinants behind each scenario's modal split.

The microeconomic optimization of the TILT model

The microeconomic component of the TILT model is based on a representative agent's optimization of decisions based on the opportunities inherent to household/firm locations (on an aggregated level), transport costs (ventilated into different categories referring to household/firm transport monetary budgets) and infrastructure availability (based on a lateness index) [START_REF] Lomax | Quantifying Congestion Final Report[END_REF].

The microeconomic component is largely inspired by developments done on ant algorithms (4 & 13) and relies on the idea of a representative agent that optimizes his/her transport choices by taking into account opportunity and cost in respect to a certain level of service on infrastructure. Thus, the results stemming from the macroeconomic determination will influence the representative agent's choices. In turn, these choices must be coherent with the overall transport structure in order to be validated and represented in the decision table where the value assigned to each choice ( a ij (t) ) is calculated using the following equation:

        i il il Ni ij ij ij N j t t t a              ) ( ) ( ) ( 1 EQUATION 1 Where: cost y opportunit ij   E Q U A T I O N 2 and ) ( ) ( t index Lateness t ij   E Q U A T I O N 3
The TILT model considers opportunities as the sum of the consumption in goods and services in a certain period of time (S. LINDER, [START_REF] Linder | The Harried Class of Leisure[END_REF]) and that the lateness index is defined by the difference existing between normal transit time and the real transit time. This last indicator is useful in factoring in speed, distance and time into the calculation and has the convenience of being comparable between modes.

In this way, TILT has been designed to be a very long-term equilibrium model by combining a macroeconomic and microeconomic structure in a backcasting approach.

The insight modules of the TILT model

Three additional modules enable the model to take into account new motor technologies and to facilitate sensitivity and impact assessments in three geographical scales: urban, regional and interregional.

The three modules are the:  vehicle fleet dynamic and technology evolution module that analyzes technological impact based on market penetration probabilities and vehicles' survival rates for different motor technologies and different transport services (road, rail, sea, air, inland waterways) [START_REF] Crozet | Comment satisfaire les objectifs internationaux de la France en termes d'émissions de gaz à effet de serre et de pollution transfrontières ? Programme de recherche consacré à la construction de scénarios de mobilité durable[END_REF].

 public policy module that joins a sensitivity analysis (for policy categories) and multicriteria analysis (for specific public policies) in order to offer a detailed impact assessment of actions on CO 2 emissions (4).

 impact assessment module (based on an input-output equilibrium analysis) that details impacts on employment and production by economic sector (4).

In short, the TILT model's structure enables the user to calculate energy consumption and pollutants emitted by transport activity (freight and passengers) on different geographical scales.

The TILT model has three important functions:  modeling passenger-kilometers and ton-kilometers coherent with a micro/macroeconomic equilibrium structure according to motor technology used for journeys and area of service  modeling the vehicle fleet according to: age, motor technology, and year of production (for freight and passengers)  modeling and assessing public policy impacts on CO 2 emissions, infrastructure investment needs as well as overall impact on the economy.

By joining these three functions and the different TILT modules in a micro/macroeconomic equilibrium structure, it is possible to build scenarios based on different values for the exogenous variables (see table 1, N.B. GDP is endogenous). These scenarios portray a desired future that is determined beforehand (backcasting) and enable us to:  quantify the consequences of transport on the environment whilst detailing the systems' structure according to behavior and organizational changes, technology used, vehicle fleet dynamics, nature of a journey and vehicle age  give a precise view of traffic by motor technology, gas consumption and emission levels for each type of transport according to service distances, type of vehicle and transport cost  build policy pathways that have different impacts in each scenario configuration and on the economy.

The 2050 Scenarios

In order to start analyzing policies and technological effects on the transport system, a business as usual reference scenario (Pegasus) was needed. Since TILT is not a forecasting model we opted to use the same mobility levels as the central scenario presented in the 2006 forecast of the Conseil General des Ponts et Chaussés (CGPC) [START_REF]Démarche Prospective Transports[END_REF]. The CGPC's "World Governance and Environmental Industry" served as our reference base from which two other scenarios (Chronos and Hestia) were built in order to test different types of policies.

On an economic level, all scenarios have roughly the same hypothesis. The differences between each scenario are linked to the transport structure where: speed/GDP elasticities, modal speeds and transport times differ. The basic characteristics of each scenario are presented in table 1. The hypothesis made for the emissions calculations for each of the scenarios take into account the current state-of-affairs in France. The most important points are that:  emission calculations are "well-to-wheel"  plug-in hybrid vehicles hit the market by 2010 and electric vehicles by 2020 (first at the urban and regional scale, and then at the interregional scale)  second generation bio-fuels represent 35% of fuel sales by 2050 and are consistent with cropland use for food  all trains are supposed electric by 2050 and electricity production is supposed to be 100% from nuclear origin (compared to 80% in 2007)  airplanes emit -in average-35% less by 2050.

Before going into the details of each scenario, we will present the main results (figures 2 & 3 and table 2) stemming from the calculations based on the aforementioned hypotheses. This will enable the reader to discover the specifics of each scenario afterwards, being already aware of the main results.

Main Results

The first graph shows France's baseline mobility level (year 2000), as well as the levels calculated for the three scenarios: Pegasus, Chronos and Hestia. As we can see, the mobility levels practically double between 2000 and 2050 for the first two scenarios (Pegasus and Chornos) whereas for the third scenario (Hestia), the mobility level grew at a slower rhythm.

FIGURE 2 Freight and Passenger Mobility in 2050

These mobility levels are associated to specific infrastructural needs and to CO 2 emission levels. The investment needed to meet the infrastructural needs are show in table 2 and are based on a per "kilometer-of-construction" cost of 5 millions euros for highways; 1 million euros for local streets; 12 million euros for railroad lines (mixed average for high speed and normal lines) and 30 million euros for public transport on dedicated lanes (mixed average for buses, subway, tramways and intercity rail). Furthermore, investment needs are totaled in actual (2000) Euros and were calculated considering an average cost per "kilometer-of-construction". The operating costs are not considered (16 & 17). For all three scenarios, we suppose that public transport infrastructure will experience an increase of 15% in overall productivity (as considered by FAIVRE D'ARCIER's study [START_REF] Faivre D'arcier | Prospective pour un financement durable des transports publics urbains (horizon 2015[END_REF]) during the 50-year period. The values obtained for the infrastructural investments needed for each scenario are comparable to those calculated by the WAELBROECK-ROCHA/BIPE report about the financing of the French transportation system to 2030 [START_REF] Waelbroeck-Rocha | Le financement du système de transport terrestre à l'horizon 2030[END_REF]. In figure 3, we can see that the three scenarios show different results concerning CO 2 emissions. The first scenario (Pegasus) represents a 48% reduction in emissions whereas the other two scenarios (Chronos and Hestia) present a reduction of, at least, 75%.

The three scenarios

In order to better understand what these results imply, it is necessary to clearly explain what is at play in each of the options. Thus, we will now present the reader with the specifics for each of the three scenarios.

Pegasus -Promoting strict technology standards

Pegasus is a BAU (Business As Usual) scenario where the speed/GDP elasticity of 0.33 for passengers and 0.6 for freight are maintained for the 2000-2050 period and where transport times are stable (1 hour per person per day). Pegasus lets us appreciate:  transport traffic in a situation where there is no major public policy affecting behavior and/or the system's regular performance (continued infrastructure investments and optimization),  the effects of new motorization technologies on total CO 2 emissions.

In this manner Pegasus lets us evaluate the contribution of strict and realistic technology standards that -according to our calculations-would lead to half of the reductions of the CO 2 emissions target. Moreover, if we suppose that it is absolutely necessary to get to the desired reduction target (-75% CO 2 emission with respect to the year 2000 baseline) solely based on new motor technologies, Pegasus shows that "zero emission vehicles" (well-to-wheel) would be required to hit the market between 2020 and 2030. Furthermore, roughly 80% of the total vehicle fleet should be "zero emission" (well-to-wheel) by 2050. This would require a paradigm shift in technology that is seemingly unrealistic (nevertheless, it could happen, as rapid technology shifts have been observed before). Furthermore, prudence dictates that one should not rely solely on new motor technologies to reach the desired emissions reduction target. This pushes us to consider other scenarios.

Chronos -Promoting green multimodality

In this scenario, market oriented policies constrain the use of fast high carbon footprint modes leading to an increase in slower and cleaner transport modes (see figure 2). In Chronos the 75% reduction objective is nearly attained (see figure 3) through an action favoring greener modes by increasing transport costs accordingly to speed and emissions (per mode). As a result, the macroeconomic model shows sharp changes in the mobility determination that, in turn, has strong implications on behavior patterns. Thus we observe:  a trade-off between the system's need for speed (coupled to growth)  an increase in transport times in order to be able to take full advantage of all modes.

In this situation, Chronos implies a speed/GDP elasticity equal to zero, which translates into an increase in transport times (roughly 1 hour and 20 minutes per person per day) because transport distances are still supposed as being coupled to growth. Since Chronos is a scenario based on market oriented public policies in an infrastructure intensive situation (because transport distances and public transport traffic keep increasing), public action in this scenario sums up to an increase in transport cost.

In Chronos, the microeconomic optimization is carried out in a carbon constrained economy, where the limits of new motor technologies push towards a differentiated modal evolution in order to further reduce emissions. This implies that transport costs go up whilst opportunities are constant.

In this manner, Chronos lets us appreciate that a mix of technology and strong policy can get us to the wished reduction target. Nevertheless, Chronos also shows that the limits of a continuous increase in transport distances are linked to financial constraints on the infrastructure investments required to ensure a steady growth in transport distances. Although investments represent the same percentage of GDP as today, we can question the viability (at least in matters of public acceptance) of large infrastructure investments (see table 2) in a situation where transport times and costs rise sharply.

Hestia -Promoting decoupling

The main issue in this scenario is a trade-off between an elevated transport cost and transport distances. Indeed, transport costs (both in time and in money) are higher than in Chronos and this translates into economic agents choosing to modify their locations and concentrate on proximity strategies (see figure 2).

In this manner, Hestia leads the way to the 75% (see figure 3) reduction objective through public policies which entail new trade-offs between location and transport distances. Thus, Hestia implies a speed/GDP elasticity equal to zero but since transport distances increase less rapidly -than in Pegasus and Chronos-transport times are reestablished around one hour per person per day. Furthermore, this new equilibrium based on proximity also gives a better opportunity for « low range 0 emissions technologies » to develop faster and better.

Higher prices and a growing degree of saturation push towards a differentiated spatial and modal evolution spawning from public action acting on opportunity and cost.

In this manner, Hestia lets us appreciate a situation where mobility increases from the 2000 level but where infrastructure needs are not as overwhelming as in the other scenarios. Furthermore, Hestia depicts a situation where although transport cost goes up, opportunities keep on increasing and transport times are lowered. What is more, this

cost   y opportunit ij  cost    y opportunit ij 
scenario boasts reduced investments needs. In that sense, we could suppose that this scenario might enjoy higher public acceptability. Nevertheless, very high constraints on air travel might overturn its attractiveness.

Assessing Public Policies and its Effects.

As we showed in the last section, the three scenarios presented imply different strategies concerning public policies. This entails big differences concerning choices in economic agents but also in the needed public investment for infrastructure and in transport spending. In order to have a better idea of what this means, a sensitivity analysis (SA) was performed on all three scenarios and results were obtained on two levels:  contribution of the different policy categories to reductions  implications on transport monetary budgets (for firms and households).

Thus, the SA of the three scenarios is calculated on the basis of effective CO 2 reductions (i.e. those that are not due to new motor technologies, which account for 48% of the reductions). The results obtained are as follows: Concerning the implication of public policies on transport monetary budgets, our analysis is carried out by keeping transport demand determinants constant in time, in order to see what new traffic and transport costs imply on this structure. We are aware that this assumption on the determinants is quite unrealistic, nevertheless it is only used to asses the implications of public policies.

The Pegasus scenario is based on an inelastic market structure largely dependent on private vehicles. In the scenario, the modal split is explained by the fact that a rise in oil prices translates into behavior changes that are determined by a sharp increase in private vehicle cost (figure 5) accompanied by a comparatively more interesting offer from public transport. Even though private vehicle costs go up, transport in Pegasus is very dependent on road transport and, thus, dependent on road infrastructures.

In Pegasus, the need for rail infrastructures is very important. Firstly, the mild increase of the cost linked to personal vehicle use incites the use of public transport (of which rail is a big part) on an urban and regional scale. Secondly, long distance travel in Pegasus is largely based on low air fares and a growing high speed train sector. Indeed, since Pegasus is based on a business as usual evolution -established on growing distances with stable transport times-high speeds on interregional transport are a big part of the scenario's equilibrium.

As we have already stated, in this scenario, the 75% reduction is not attained. The reductions in CO 2 emissions are solely obtained through the introduction of new hybrid motor technologies. In this manner, public policies appear merely as accompanying policies in an ongoing trend of the system with no real impact on GHE emissions. This explains why the scenario is mostly linked to regulation oriented policies as well as infrastructure and spatial development policies that accompany the BAU development. In the Chronos scenario, constraints on speed and emissions come into play as a signal aiming at changing behavior patterns. In consequence we observe an increase in road transport costs (figure 5) that are coherent with the CO 2 values that are actually being proposed by public authorities (A 32€ tax per ton of CO 2 is supposed to bring in over 8 billion Euros in tax revenues by 2010. This tax is supposed to go up to 350€ by 2050). This rise in road transport cost translates into a sharp increase in the use of rail and public transport for personal and freight mobility. This in turn implies that average speed in the system goes down and transport times go up. This means that, if we suppose that the market structure and its inherent elasticities do not change in time, household and firm transport budgets would invariably go up in a very sharp way (more or less depending on car use elasticity).

In Chronos, the monetary budgets for road transport go up so much that they are over 15% of total household and firm budgets (which are known to be fairly stable in time) [START_REF] Schafer | The future mobility of the world population[END_REF]. Therefore, in order for Chronos to be viable, we should accept a very sharp increase in public transport and rail subsidies (as a return of tax revenues into the system that would ease monetary budgets); accept that transport monetary budgets will go over the 15% mark which supposes that demand determinants and market structure would strongly change (especially concerning personal vehicle elasticity).

The system's adaptation in Chronos should be accompanied by very big investments in rail infrastructures that will offer over 30% of total trips. This entails a very big need in investment needs but also a very big need in subsidies for public transport.

Hestia is a scenario that is very influenced by proximity services, the public policies in play are largely related to spatial planning and infrastructure investment. This is also explained by the fact that Hestia is a scenario where market oriented policies are in place.

In the Hestia logic, the main trade-off at play is directly linked to location strategies and production organization aimed at decoupling transport distances. This entails a densification of main cities and production sites which would in turn translate into a sharp increase in the use of urban and regional road networks. In this manner road investments needed are less important than in other scenarios but they are totally concentrated on urban and regional infrastructure.

Even though road transport is important on an urban and regional level, investments in public transport represent over 30% of total mobility, especially on an interregional scale. This is automatically translated in very big needs in infrastructure investment on rail. Unlike Chronos, this scenario's characteristics point towards a more stable monetary budget (around 15%), although current mobility determinants would imply a very big increase in road transport costs followed by a very sharp decrease in rail and public transport costs.

In this manner, the fact that public policies at play in the Hestia scenario aim at reducing CO 2 emissions through the decoupling of transport distances translates into investments needs that are largely inferior to Pegasus and Chronos.

Lastly, although specific results were not presented for the economic impact of each scenario, the analysis carried out on this matter shows that no negative impact on the economy is to come from any of the scenarios if -and only if-tax revenue stemming from carbon taxing is correctly reinvested in transport services (infrastructure, regional transport aid, etc.) This joins the opinions of the recent CO 2 tax workgroup formed by French public authorities that even a positive impact on the economy could be expected if funds coming from tax revenues are correctly allocated.

Conclusions

On the basis of the three LET-ENERDATA scenarios that depict three different ways of attaining planned CO 2 reductions, we have presented new insight concerning public policies, investment needs and microeconomic trade-offs linked to sustainable transport development. What is more, this analysis was carried out with a methodology and a model that use widely available national statistics and that are applicable to other countries.

In sum, realistic technological hypothesis show that a 50% reduction in emissions is a clear possibility and that going further -solely based on new technologies-would require very big advances in zero emission vehicles.

Nevertheless, in the absence of these new technologies, the remaining reductions in emissions are possible through different types of policy mixes that come down to:

 encouraging important modal shifts that would translate into a decrease in total average speed which would in turn make transport times go up  encouraging modal shift accompanied by a decoupling of transport distances, consecutively, this would help to maintain stable transport times.

This implies public action based on the optimization of cost and opportunities (subject to carbon constraint) aiming at a differentiation in modal and spatial evolution on the basis of the carbon footprint.

No real way of implementing these policies was proposed because the focus of this paper was not to offer a ready-made policy-mix. However, we offer a complete view of current studies concerning organizational solutions that could lead to a reduction in oil consumption and emissions through important changes in the transport structure and behavior patterns.

Although it is safe to say that organizational changes will most certainly be a part of the future of transport, the real challenge for a sustainable future will be linked to the analytical basis and decision-making tools used to plan policies for a sustainable environment.

In this manner, a shift from "truth seeking" methodologies referring to the maximization of opportunities based on quantity and variety towards "path seeking" methodologies referring to a differentiation of choices based on optimizing cost and opportunities is necessary.

Moreover, the efficiency of environmental policies in transport will lie on the ability of international deciders and planners to convince almost every country in the world of the necessity for action in tackling CO 2 emissions while not forgetting equity issues between countries and populations.

FIGURE 1

 1 FIGURE 1 TILT Model Structure

FIGURE 3

 3 FIGURE 3 Freight and Passenger GHG Emissions in 2050

FIGURE 4

 4 FIGURE 4 Public Policy Sensitivity AssessmentPublic Policy Sensitivity Assesment

FIGURE 5

 5 FIGURE 5 Transport Monetary Budget Distribution for Scenario Modal Split -Pegasus-2050

TABLE 1 11 FIGURE 1 6 FIGURE 2 10 FIGURE 3

 111162103 FIGURE 1 TILT Model Structure ....................................................................................... 6 FIGURE 2 Freight and Passenger Mobility in 2050 .......................................................... 10 FIGURE 3 Freight and Passenger GHG Emissions in 2050 .............................................. 11 FIGURE 4 Public Policy Sensitivity Assessment .............................................................. 14 FIGURE 5 Transport Monetary Budget Distribution for Scenario Modal Split -Pegasus-2050.................................................................................................................................... 15

TABLE 1 Scenario Characteristics Scenario Characteristics URBAN

 1

	Freight	Km/h
		-Road urban	50	60	52	52
	Passengers	Km/h
		-Private car urban	23	30	25	25
		-Public transport urban	20	24	20	22
	REGIONAL	Km/h
	Freight	-Road regional	50	60	52	52
		Km/h
	Passengers	-Private car regional	58	67	58	55
		-Public transport regional	58	68	57	54
	INTERREGIONAL	Km/h
	Freight	-Rail+Plane national	40	63	45	45
		-Rail+Plane international	-	7 0	7 0	7 0
		Km/h
	Passengers	-Private car interregional	110	115	90	90
		-Public transport interregional	80	90	80	80
		-High speed rail interregional	250	250	250	250
		-Plane	500	500	500	500

TOTAL Freight (nat/inter) Km/h 43 54/52 43/52 43/52 Passengers Km/h 45 50 37 37

			Pegasus	Chronos	Hestia
		2000	2050	2050	2050
	Elasticities
	Speed/GDP	-	0 , 3 3	0	0
	T.Km/GDP	-	0 , 6	0 , 6	0 , 3
	T.Km/International trade	-	1,6	1,6	0,25
	Macroeconomics
	Population	64	67	67	67
	Average Yearly GDP Growth		1,5	1,5	1,5
	Child per household	2,19	2,15	2,15	2,15
	Productivity rate	100	225	225	225
	Transport Time Budget	1	1	1 , 2	1
	Note: -means not applicable			H.G. LOPEZ-RUIZ 2009

TABLE 2 Investment needs for infrastructures

 2

		2050	Per	% of	2050	Per	% of	2050	Per	Year	% of
	Billions of € Mode	Pegasus	annum	GDP	Chronos	annum	GDP	Hestia	annum	2007	GDP
	Investments Road	1043	21	0,7%	384	8	0,3%	140	3	12	0,9%
	Rail	747	15	0,5% 1529	31	1,1%	992	20	2	0,2%
	Public Trn. 137	3	0,1%	74	1	0,1%	77	2	2	0,2%
	Others	-	-	-	-	-	-	-	-	1	-
	Total	1927	39	1,4% 1987	40	1,4% 1209	24	18	1,4%
	H.G. LOPEZ-RUIZ 2009
	Note: -means not applicable / values are in 2000€

Transport Monetary Budget Distribution for Scenario Modal Split

	H.G. LOPEZ-RUIZ 2009										Urban VP road -
												Passengers
												Regional VP road-
												Passengers
												Interregional VP road -
	Percentage of Total Transport Budget	Pegasus Chronos Hestia										Passengers Urban PT road-Passengers Regional PT road -Passengers Interregional PT road -Passengers Freight road Urban PT rail -Passengers Regional PT rail -Passengers Freight Rail Interregional PT rail -Passengers Freight IW HST interregional -Passengers Plane -Passengers
		0%	10%	20%	30%	40%	50%	60%	70%	80%	90%	100%

The authors wish to acknowledge the work of Bertrand Château (ENERDATA) and Vincent Bagard (University of Lyon -ENERDATA)


Monica Spinu

Nuno Dos Santos

Nicolas Le Moigne

Patrick Navard
email: patrick.navard@mines-paristech.fr

Dos Nuno

Nicolas Santos

Patrick Le Moigne

Navard

How does the never-dried state influence the swelling and dissolution of cellulose fibres in aqueous solvent?

Keywords: Cellulose fibres, swelling, dissolution, never-dried state, Nmethylmorpholine-N-oxide, sodium hydroxide

The swelling and dissolution capacity of dried and never-dried hardwood and softwood pulps and cotton linters was compared in two aqueous solvents, Nmethylmorpholine-N-oxide (NMMO)-water at 90 °C with water contents ranging from 16 to 22 % and NaOH -water at -6 °C with NaOH contents ranging from 5 to 8 %. Swelling and dissolution mechanisms were observed by optical microscopy and dissolution efficiency was evaluated by recovering insoluble fractions. The results show a contrasted picture towards the effect of the never-dried state on the swelling and the dissolution capacity depending on the origin of the fibres and the type of aqueous solvent. In the case of NMMO -water, the presence of water within and around the fibre does not seem to favour dissolution initiation but after two hours of mixing the dissolution yield appears to be similar for either dried or never-dried state.

The limiting factor for dissolution in NMMO -water is not the penetration of the solvent inside the cellulose fibres, but only the local concentration of NMMO molecules around the fibre. For NaOH -water, both optical microscopy observations on individual fibres and dissolution yield measurements show that the never-dried state is more reactive for softwood pulps and cotton linters and has no significant effect on hardwood pulps. In this case, the local decrease of solvent strength is counteracted by the opening of the structure in the never-dried state which should enable the Na + hydrated ions to penetrate easier.

Introduction

Wood is a porous material which contains large amounts of free and bound water, up to 50-60 % of the total weight of timber depending on many factors, including botanic origin and environmental conditions. After wood chipping and the various aqueous chemical treatments used to prepare cellulose pulp, wood cellulose fibres are wet, i.e. containing 50 to 70 % of water. For integrated pulp and paper or dissolving pulp and regenerated cellulose mills, these wet or never-dried fibres offer the advantage of being very flexible and easily conformable, as compared to the same fibres having been dried. For use in non integrated mills, fibres need to be dried before they are shipped to customers.

Never-dried fibres have several key properties different from dried fibres. They are in a swollen and more accessible state. For example, studies performed on fibres in a never-dried state showed that they can absorb much more chemicals than fibres in their dried state [START_REF] Brännvall | Aspects on strength delivery and higher utilisation of the strength potential of softwood kraft pulp fibres[END_REF][START_REF] Gerber | Adsorption of hemicellulases onto bleached kraft fibers[END_REF][START_REF] Ibbett | Solute adsorption and exclusion studies of the structure of never-dried and re-wetted cellulosic fibres[END_REF]). After drying and rewetting, fibres change irreversibly their properties; the internal volume of the fibre shrinks resulting in a decrease of the water retention value (WRV). This phenomenon is due to changes that occur at the structural level and was named hornification by [START_REF] Jayme | Mikro-quellungsmessungen an zellstoffen[END_REF]. [START_REF] Robertson | Some observations on the effects of drying papermaking fibres[END_REF] and [START_REF] Maloney | Intra and inter-fibre pore closure in wet pressing[END_REF] showed further that besides drying and rewetting, hornification can also occur during wet pressing. Since the earliest observations of [START_REF] Jayme | Mikro-quellungsmessungen an zellstoffen[END_REF], a lot of attempts have been made to explain the causes and the mechanisms involved in hornification. Most of the existing literature about hornification refers to cellulose fibres used in paper industry, this phenomenon having undesirable effects on paper strength properties. [START_REF] Thode | Dye adsorption on wood pulp, IV. Note on effect of drying of pulp on specific dye adsorption[END_REF] showed by dye adsorption experiments a reduction of surface area of sulphite pulps due to hornification. It was demonstrated further that the structural change responsible for hornification is the irreversible pores closure that occurs upon drying [START_REF] Stone | A structural model for the cell wall of water-swollen wood pulp fibres based on their accessibility to macromolecules[END_REF]. By solute exclusion method, [START_REF] Stone | A structural model for the cell wall of water-swollen wood pulp fibres based on their accessibility to macromolecules[END_REF] showed that the large and intermediate size pores (2 to 20 nm) are reduced upon drying. Based on NMR studies, [START_REF] Maloney | Intra and inter-fibre pore closure in wet pressing[END_REF] concluded that hornification induces the closure of larger pores. The larger pores within Kraft fibres were also considered by [START_REF] Park | Changes in pore size distribution during the drying of cellulose fibers as measured by differential scanning calorimetry[END_REF] and [START_REF] Wang | Improving the papermaking properties of kraft pulp by controlling hornification and internal fibrillation[END_REF] as being the earliest to close upon drying. As discussed by Fernandes [START_REF] Fernandes Diniz | Hornification -its origin and interpretation in wood pulps[END_REF], the possibility to reverse hornification was also investigated by several authors.

A mechanism proposed to explain hornification is the reorganisation of the fibre cell wall structure. According to [START_REF] Clark | Pulp technology and treatment for paper[END_REF], when adjacent surfaces of cellulose and hemicelluloses previously separated come into contact upon drying, it results in the creation of additional crystalline zones that prevent the separation of the two surfaces. [START_REF] Oksanen | The role of hemicelluloses in the hornification of bleached kraft pulps[END_REF] demonstrated further that the removal of xylan and glucomannan is responsible for the decrease of the WRV of dried kraft pulps and suggested that hemicelluloses present in the fibre pores and in the interfibrillar spaces hamper hornification. Many research teams mentioned "hydrogen bonding" as being the main mechanism responsible for the failure of pores to re-open when the fibres are dried and rewetted [START_REF] Kato | A review of the relationship between thermally-accelerated ageing of paper and hornification[END_REF][START_REF] Lindström | The porous lamellar structure of the cell wall[END_REF]). An alternative crosslinking mechanism is supported by several workers. [START_REF] Back | Thermal auto-crosslinking in cellulose material[END_REF] and [START_REF] Ruffini | Improvement in bonding of wood pulps by the presence of acidic groups[END_REF] suggested that thermal crosslinking takes place through the formation of esters. This was supported further by Fernandes Diniz et al.

(2004) who assumed that hornification is due to the formation of lactone bridges, i.e. cyclic ester between hydroxyl and carboxylic acid groups, between adjacent cellulose chains.

Differences between the never-dried and the dried states also occur for cotton fibres. In the boll, cotton fibres grow in a close water-containing system and they dry at the end of the growth, upon opening of the boll. Never-dried cotton fibres extracted from the bolls exhibits higher water absorbancy compared to dried ones. Besides, age (days after post-anthesis) is strongly influencing the properties of cotton fibres. Significantly higher contents of bound water were measured at early stages post-anthesis [START_REF] Mizutani | Water absorbancy of never-dried cotton fibers[END_REF].

Never-dried young cotton fibres indeed have thinner secondary walls and larger lumens which allow higher accumulations of bound water but involve weaker tensile strengths and lower crystallinities [START_REF] Tsuji | Properties and structure of never-dried cotton fibers. III: Cotton fibers from bolls in early stages of growth[END_REF]) as compared to older ones. Dried and never-dried fibres have thus different properties that influence their behaviour.

As known cellulose does not melt before it thermally degrades, this causes difficulties in its processing. Up to now, the only way to prepare a thermoplastic or highly soluble form of cellulose is to either derivative cellulose to materials like cellulose acetate, or to dissolve cellulose in suitable solvents, the latter being employed in film extrusion and fibre spinning. The solubility of native cellulose fibres is usually not very high, and complete dissolution up to the molecular level is very difficult to achieve [START_REF] Röder | The influence of activation on the solution state of cellulose dissolved in N-methylmorpholine-N-oxide-monohydrate[END_REF]; this is ascribed to limited pore accessibility, long chain length, high crystallinity, the presence of hydrogen bonds or influence of other molecules like hemicelluloses or lignin [START_REF] Klemm | Comprehensive cellulose chemistry, Volume 1: Fundamentals and analytical methods[END_REF]. Cellulose substrates are often activated, i.e. submitted to various chemical, physical or mechanical treatments to increase their reactivity and solubility.

Dissolution is classically studied using dried fibres as the starting material. Synthesis of cellulose derivatives at the industrial scale is also usually achieved starting from the dried state. Since the structure and properties of never-dried fibres are different from dried fibres, the question we address here is whether the never-dried state of a fibre increases or decreases its solubility in an aqueous solvent. On one hand, the never-dried state is a more open state that should favour penetration of solvent and accessibility to cellulose chains. On the other hand, the presence of water inside and around the fibres should change locally the quality of aqueous solvents by diluting them and thus decreasing their strengths. This paper reports an investigation of the swelling and dissolution capacity of dried and never-dried wood cellulose and cotton linter fibres in two aqueous solvent solutions of N-methylmorpholine-N-oxide and NaOH. Two complementary techniques will be used: firstly, observing the swelling and dissolution mechanisms by optical microscopy and secondly measuring the dissolution yield using a centrifugation method. The first method gives indications on the way cellulose fibres swell and dissolve but has the drawback of considering only a few fibres. The second method measures the average dissolution yield over several thousands of fibres.

Materials and Methods

Cellulose fibre samples

Nine different dried or never-dried cellulose fibres were provided by several companies. Their botanic origin, treatment and main properties are listed in Table 1.

Conditioning of the samples and drying procedure

The never-dried pulps and the cotton linters were received in a bulk form packaged in sealed hermetic bags, while the dried wood pulps were received in the form of dried pulp sheets. In order to facilitate the processing of the dried fibres, the pulp sheets had to be disintegrated. The pulp was first soaked in water for 15 minutes, defibrillated with a disintegrator for 5 minutes at 500 rpm. After removing most of the water with a centrifuge, the pulp was acclimatized in a room with controlled atmosphere (20 ºC and 65 % humidity), reaching a consistency of ~93 %, then packaged in sealed hermetic bags. The initial moisture content of all samples before experiments was determined by measuring the weight of a sample portion before and after drying overnight in a vacuum oven at 70 ºC and 50 mbar in presence of silica gel.

In order to study the influence of the water content, never-dried samples S-03, S-05, S-06 and S-07 were dried for different periods of time (from 0-70 minutes) in an air oven at constant temperature (50 °C). The water quantity was calculated by measuring the weight of the samples prior to drying and at different drying times and the samples were then directly packaged in sealed hermetic bags. The mass of sample as a function of time in an oven at 50 °C is given on Figure 1 for samples S-03, S-05, S-06 and S-07. This allows defining what are called partially-dried samples and dried samples, as shown on the Figure. In the never-dried and partially-dried state (from 0 to 30 min of drying), the samples still contain some amount of bound and free water, especially for the never-dried samples. In the completely dried state (after one hour of drying at 50 °c), the samples contain only some bound water [START_REF] Maloney | Removal of water from the cell wall during drying[END_REF][START_REF] Nakamura | Studies on bound water of cellulose by differential scanning calorimetry[END_REF].

Aqueous solvent systems

Two aqueous solvents were used, N-methylmorpholine-N-oxide (NMMO) and sodium hydroxide (NaOH) both mixed with distilled water. The mixtures based on NMMO with different water contents were prepared by adding distilled water to the anhydrous NMMO powder (97 % purity, provided by Sigma Aldrich). Four water concentrations were used, respectively 16 %, 18 %, 20 % and 22 % (w/w). All dissolution experiments with NMMO-water were carried out at 90 °C. The mixtures based on NaOH were prepared by mixing distilled water with NaOH (98 % purity, provided by Sigma Aldrich).

Four different mixtures were prepared with NaOH concentrations of 5 %, 6 % and 8 % w/w. All experiments with NaOH mixtures were carried out at -5 -6 °C.

Optical microscopy observations

Optical microscopy observations were carried out after placing the cellulose fibres between two glass plates. The solvent was put in contact with the fibres by introducing it between the two plates by capillary forces. No agitation was applied to the system. The mechanisms of swelling and dissolution were observed in transmission mode using either a Metallux 3 (Leitz) or a Leica DM 4500P equipped with a Linkam TMS 91 and a Linkam TMS 94 hot/cryo stage respectively. Images were obtained with a high resolution 3-CCD camera (1360 x 1024 pixels) JVC KY-F75U, recording digitally the observations with the Archimed 5.5.1 software from Microvision Instruments. Swelling and dissolution trials were performed several times by different experimenters in order to check the reproducibility and the accuracy of the results.

Dissolution yield measurements

NaOH-water: 2 g of cellulose fibres (dry weight) were added to 66 g of distilled water, gently stirred and stored at 4 ºC for 1 hour. The water content of the samples was taken into account when calculating the total amount of distilled water to be added. 132 g of NaOH 12 % aqueous solution were prepared and stored in a ethylene glycol bath at -6.1 ºC for 2 hours. Both preparations were mixed together, giving a solution of 1 % cellulose in 8 % NaOH aqueous solvent. The solutions were stirred using a rotary overhead mixer during 2 hours at -6.1 ºC and ~1000 rpm. The resulting solutions were then centrifuged to separate by decantation the insoluble fraction from the clear solution fraction. The centrifugation was done at 9000 rpm (equivalent to 9050 g) for 8 min with a Hettich Universal 320RHK centrifuge equipped with a 1620A rotor and thermo-regulated with an external Julabo cryostat set at 0 ºC.

The recovered insoluble fractions were washed with 1 L of distilled water in a sintered glass filter G3 and dried overnight in a vacuum oven at 70 ºC and 50 mbar in presence of silica gel. After drying, the insoluble fractions were weighed and the dissolution yield, i.e. the amount of material contained in the clear solution fraction, determined. NMMO-water: the same procedure was followed except that the solution was a 1% cellulose / NMMO -18 % of water (w/w) mixture. The total weight of the mixture including the cellulose sample was 50 g, the temperature of mixing was 90 °C and the temperature of centrifugation was 75 °C.

Results and discussions

The dissolution of cellulose can be assessed through different complementary ways like looking at the state of dissolution by light scattering or NMR or measuring dissolution kinetics. An interesting method that can be applied to cellulose fibres is to look at the way the fibres are swelling and dissolving. With very good solvents as NMMO-water solutions containing 16 % of water (w/w), the fibres are disintegrated into elongated fragments in a rather short time, with the subsequent dissolution of all the fragments (Cuissinat and Navard 2006a). When the quality of the solvent decreases (NMMO-water mixtures containing 20 -22 % of water (w/w)), the primary wall, if present, is not easily dissolved. The inner zones of the fibre dissolve first (Le [START_REF] Moigne | Gradient in dissolution capacity of successively deposited cell wall layers in cotton fibres[END_REF]Le Moigne et al. 2010a) and the fibre adopts a balloon morphology. With time, the whole fibre dissolves. For poorer solvents, there is a ballooning morphology with dissolved cellulose inside, but the whole fibre does not dissolve and stays in this ballooned state. This was observed in the case of aqueous solutions containing 7.6 % of NaOH at -5 °C (Cuissinat and Navard 2006b). With a further decrease of solvent quality, the fibre swells slightly, swelling homogeneously without dissolving. This description of swelling and dissolution mechanisms as a function of solvent quality has a general character for fibres coming from different botanical sources and even for cellulose derivatives that have been prepared in a heterogeneous way (Cuissinat and Navard 2008;Cuissinat et al. 2008).

What will be investigated here is the influence of the water content of the fibre on the swelling and dissolution mechanisms, especially in the neverdried state. The main effect to be expected comes indeed from the never-dried state, where fibres are in a large "open" state. As written in the introductory part of this paper, chemicals easily penetrate the fibre when it is in the neverdried state. However, the presence of water within and around the fibres should decrease the quality of the solvent. We will examine whether the never-dried state improves the reactivity towards swelling and dissolution or not, at least in the two aqueous solvents that are considered here.

Swelling and dissolution in NMMO-water

The optical microscopy observations of the dried fibres in water mixtures showed two mechanisms: fragmentation followed by full dissolution for the 16 and 18 % water concentrations and ballooning followed by full dissolution for 20 and 22 % water concentrations. These results are reported in Table 2 for the "D" case, i.e. dried fibres. As assessed by optical microscopy, NMMO -16 and 18 % water were good solvents for the four cellulose samples (S-03, S-05, S-06, S-07 after drying) with a fragmentation mechanism while NMMO-20 and 22 % water was not so good with a slower dissolution by ballooning.

At 16 % water, the partially and never-dried state did not change the dissolution mechanism which was still fragmentation for all pulps. In NMMO -20 and 22 % water, the optical observations of the partially and never-dried fibres showed that the dissolution mechanism was not affected by the presence of some water within and around the fibres which still dissolved by ballooning.

An effect of the presence of water was visible for the intermediate concentration, NMMO -18% water. In the dried state, only dissolution by fragmentation was observed, whereas both fragmentation and ballooning were observed in the partially-dried state for all pulps. In the never-dried state, both fragmentation and ballooning were observed for S-03 and S-05 pulps and fibres dissolved by ballooning for S-06 and S-07 pulps which means poorer dissolution for the latter samples as compared to S-03 and S-05 pulps. In NMMO -18% water, there is thus a switch from the dissolution by fragmentation for dried fibres to the dissolution by ballooning for never-dried fibres (Figure 2a and2b). Therefore, water within and around the fibre decreases the dissolution efficiency. It has to be noticed that the difference of behaviour between the never-dried S-03, S-05 and S-06, S-07 pulps illustrates the influence of the severity of the sulphite pre-treatment. The highest intrinsic viscosity pulps (S-06, S-07) were in fact digested at the highest kappa number which involved less degradation of the walls and the cellulose chains as compared to the two other pulps (S-03, S-05). Consequently, the dissolution is easiest for S-03 and S-05 pulps for which the original solid state is much more damaged as compared to S-06 and S-07 pulps.

Dissolution yield measurements after centrifugation were performed on solutions of never-dried softwood bleached sulphite pulp S-03 and dried S-04 in a NMMO -18 % water mixture. In the experimental protocol used for optical microscopy observations, the water brought by the never-dried fibres had no significant influence on the total amount of water, since only a few fibres were placed in a large amount of solvent. It was not the case for the preparation of the 1 % cellulose solutions for subsequent dissolution yield measurements and care was exercised for taking into account the water brought by the never-dried fibres in the calculation of the total water content of the solvent. The temperature during mixing was 90 °C and the temperature of centrifugation was 75 °C to prevent the crystallisation of the NMMO -water solvent. The dissolution yields determined after centrifugation and drying of the insoluble fractions were very similar, 93.7 % and 92.1 % for the neverdried (S-03) and the dried (S-04) samples respectively (Figure 3). Optical microscopy observations of the insoluble fractions showed some unswollen fibres and some others swollen by ballooning or swollen homogeneously (Figure 4).

To conclude, the dissolution initiation is slightly slowed down in the never-dried state as compared to the dried state, especially in NMMO -18% water. It seems that the presence of water within and around the fibres pushes the dissolution mechanisms towards the one seen with a solvent containing a higher water content. It is recalled that the sequence of swelling and dissolution mechanisms can be classified into four main dissolution modes for wood and cotton fibres as a function of the quality of the solvent by using NMMO with various amounts of water (Cuissinat et al. 2006a). On the other hand, the dissolution yield measurements showed that after two hours of mixing in NMMO -18 % water, no significant influence of the never-dried state can be observed. The limiting factor for dissolution in NMMO -water is not the penetration of the solvent inside the cellulose fibre, which is supposed to be easier in the never-dried state. In NMMO aqueous solvent, the only influencing parameter is the local concentration of NMMO molecules around a fibre.

Swelling and dissolution in NaOH-water

The swelling and dissolution of cellulose fibres in 8 %, 6 % and 5 % NaOH aqueous solutions was measured with dried and never-dried pulps. The dissolution mechanisms observed by optical microscopy are described below.

In NaOH 8 % -water upon cooling from 20 ºC to -6 ºC, the never-dried hardwood bleached sulphite pulp fibres (S-01) started swelling at around 10 ºC, this mechanism being more intense at -4 ºC. The dried fibres (S-02) had a slightly slower swelling initiation than the never-dried fibres. In both cases, it was observed that the parenchyma cells were not affected by the solvent.

Fibres which were highly fibrillated swelled very fast, while intact fibres showed slight swelling and ballooning in some dislocations. This can be explained by the fact that in these dislocations, the macrostructure of the fibre was deformed and damaged, allowing locally easy diffusion of the solvent into the fibre. The vessels unfolded and distended their structures but neither swelling nor dissolution were observed.

For softwood bleached sulphite pulp, in the never-dried state (S-03) at around 16 ºC the fibres started moving in space by distending themselves, and by a temperature of ~10 ºC, swelling by ballooning started (Figure 5a). For the dried fibres (S-04), the same was observed, but the phenomenon was slower. In both states, several fibres showed a large homogeneous swelling (Figure 5a) revealing the absence of primary wall as observed by Le Moigne and Navard (2010) for wood pulps and after the enzymatic peeling of the primary wall (Le Moigne et al. 2010b). As for hardwood, the parenchyma cells were not affected by this solvent system and the most intact fibres showed only slight swelling and ballooning localized in the dislocations.

Cotton linter fibres became highly swollen (Figure 5b) and ballooning was not as clearly seen as for wood pulps. Both never-dried (S-08) and dried fibres (S-09) started distending in space at ~0 ºC, followed by swelling but without dissolution. These mechanisms started faster in the never-dried state (S-08).

By decreasing the NaOH concentration to 6 %, the same swellind and dissolution patterns as with 8 % NaOH aqueous solution were observed for all samples but they occured more slowly. With a concentration of 5 % NaOH, the solvent system was not able to achieve any swelling or solvating activity for the samples used (except in the case of extremely fibrillated and thin fibres).

The following main observations and conclusions can be drawn (see Table 3). The fibres start swelling and ballooning at about 10 ºC for wood pulps and about 0 ºC for cotton linters upon cooling the fibre-solvent mixture.

The parenchyma cells, present in hardwood, are not affected by the solvent and the vessels are distending, showing an apparent swelling, but do not dissolve.

The mechanisms of swelling are different when comparing softwood and cotton linter fibres, the ballooning being more evident in softwood fibres, while for cotton linters the swelling is homogeneous along large sections (Figure 5a and b). In contrast with NMMO-water solvents, the never-dried state improves significantly the swelling initiation of the softwood fibres and the cotton linters in NaOH-water.

To complement these results, dissolution yield measurements were performed for each sample in an 8 % NaOH aqueous solution. The results are shown on Figure 3. They are in agreement with the optical microscopy observations. No significant effect is measured for hardwood pulps S-01 and S-02, possibly due to their more compact structure which is less accessible to water and also less reactive to NaOH -water mixture. For softwood pulps and cotton linters, the dissolution yield was significantly higher for the never-dried state than the dried state. It has to be emphasized that the clear solution fraction recovered after centrifugation should be composed of cellulose and hemicelluloses, the latter being present in relatively low contents in sulphite pulps.

One hypothesis to explain the highest swelling and dissolution capacity of never-dried softwood pulps and cotton linters in 6 -8 % NaOH solvent is that Na + hydrates have primary and secondary solvation cages so large [START_REF] Kunze | Na-NMR spektroskopische untersuchungen zur hydratation von natriumhydroxid in wäbriger lösung[END_REF][START_REF] Yamashiki | Some characteristic features of dilute aqueous alkali solutions of specific alkali concentration (2,5mol/l) which possess maximum solubility power against cellulose[END_REF]) that they cannot penetrate the smallest fibre pores, except if the structure is more open and full of water, as it occurs in the never-dried state. The decrease of solvent strength due to the local excess of water within and around the fibres is thus not preponderant in NaOH-water solvent unlike the NMMO-water solvent.

Conclusions

Our results show a contrasted picture towards the effect of the neverdried state on the swelling and dissolution capacity depending on the origin of the cellulose fibres and the type of aqueous solvent. The never-dried state, although being more accessible, is not necessarily the best state to swell and dissolve cellulose fibres. In the case of NMMO -water, the never dried-state does not seem to favour dissolution initiation but has no influence on the final dissolution yield. If correct, it means that the limiting factor for dissolution in NMMO -water is not the penetration of the solvent inside the cellulose fibre.

What is acting first is the fact that water around and inside the fibre dilutes the NMMO -water system locally, decreasing then its solvating activity. In NMMO aqueous solvent, the only influencing parameter is thus the local concentration of NMMO molecules which seem to be able to approach easily cellulose backbones, whatever the state of the structure of cellulose fibres, i.e. open or not. In the case of NaOH-water, both optical microscopy observations on individual fibres and dissolution yield measurements show that the neverdried state is more reactive for softwood pulps and cotton linters and has no effect on hardwood pulps. We postulate that one of the limiting factor for swelling and dissolution in NaOH-water is the size of the Na + hydrated ions, too large to penetrate inside the small pores of dried cellulose fibres, but able to penetrate easier within fibres in the never-dried state which offers a more open structure.

Fig. 1

 1 Fig. 1 Evolution of the weight of softwood samples S-03, S-05, S-06 and S-07 as a function of time upon drying in an air oven at 50 °C.

Fig. 2

 2 Fig. 2 Observations by optical microscopy of (a) dissolution by fragmentation in NMMO -18 % water of a softwood fibre (S-06) in the dried state; (b) dissolution by ballooning in NMMO -18 % water of a softwood fibre (S-06) in the never-dried state.

Fig. 3

 3 Fig. 3 Dissolution yields of the never-dried and dried pulps in NaOH 8%water and NMMO -18% water. HBSP stands for Hardwood Bleached Sulphite Pulp (samples S-01 and S-02), SBSP for Softwood Bleached Sulphite Pulp (samples S-03 and S-04) and CL for Cotton Linters (samples S-08 and S-09).

Fig. 4

 4 Fig. 4 Observations by optical microscopy of the undissolved fraction obtained after centrifugation of a solution of never-dried S-03 pulp in NMMO -18% water.

Fig. 5

 5 Fig. 5Observations by optical microscopy of (a) never-dried softwood fibres (S-03) and (b) dried cotton linter fibres (S-08) swelled in NaOH 8% -water. A homogeneous swelling is seen for cotton linter fibres while the ballooning is more pronounced for the softwood fibres. A highly homogeneously swollen softwood fibre is also visible (black arrow).

Fig. 1

 1 Fig. 1

  Fig. 2

Fig. 4 Fig. 5

 45 Fig. 4

Table 1 :

 1 List of cellulose fibres used in the study

	Name	Description	State	Moisture (%) Intrinsic viscosity (ml/g)	Source
	S-01 Hardwood (beech) bleached sulfite pulp never-dried	52	530	Lenzing AG
	S-02 Hardwood (beech) bleached sulfite pulp	dried	7	530	Lenzing AG
	S-03 Softwood (spruce) bleached sulfite pulp never-dried	75	620	Borregaard Schweiss AG
	S-04 Softwood (spruce) bleached sulfite pulp	dried	6	620	Borregaard Schweiss AG
	S-05 Softwood (spruce) bleached sulfite pulp never-dried	70	750	Borregaard Norway
	S-06 Softwood (spruce) bleached sulfite pulp never-dried	70	900	Borregaard Norway
	S-07 Softwood (spruce) bleached sulfite pulp never-dried	70	1700	Borregaard Norway
	S-08 Cotton linter		never-dried	35	700	Milouban (M.C.P.) LTD
	S-09 Cotton linter		dried	7	700	Milouban (M.C.P.) LTD

Table 2 :

 2 Summary of the mechanisms responsible for the dissolution of the fibres in NMMO with water mixtures -ND refers to never-dried state, PD to the partially-dried state and D to the completely dried state. "ball" refers to the ballooning phenomenon and "frag" refers to the fragmentation phenomenon.

	"disso" is for dissolution.

Acknowledgements. The authors want to thank Borregaard, Lenzing AG and Milouban LTD for providing the samples. NLM thanks the Carnot Mines institute for its financial support.

NDS thanks Viskase ® , Spontex ® , Sappi, Tembec, Inc. and Lenzing, AG for financial support and vTI/Universität Hamburg for academic support.