r - Convert several SpatialPointsDataFrames to raster

There are 100 SpatialPointsDataFrames in my Workspace. I loaded them on this way:

filenames <- list.files(path="",
                        pattern="XYhectareTravelTimes_ez+.*shp")

for(i in filenames){
              filepath <- file.path("/",i)
              assign(i, readShapePoints(filepath))

They are called like this:

XYhectareTravelTimes_ez10.*shp 

XYhectareTravelTimes_ez11.*shp 
XYhectareTravelTimes_ez12.*shp 

etc.

How can I convert them to rasters looping through the Workspace?

I am very new R user and hoping to find some help. thanks a lot.